Is there a rule to understand how the group "Cha" has to be pronounced?
"Character" sounds with a hard first syllable, while "Charm" sound softer, but I don't find how to tell which sound to use before earing someone saying the word. It could be because of the double consonant "rm" vs "ra"? Or is it just a matter of knowing the rule for every single word?
EDIT: More specifically, let's talk about UK English pronunciation.
I already dealt with <GH> pronunciation variation here; <CH> is a more interesting situation because it involves borrowings from familiar European languages, rather than languages written in other orthographies.
The grapheme <C> goes all the way back to the Semitic glyph gimel, the third letter of the original alphabet: 'aleph 'cow', beth 'house', gimel 'camel', etc. 'Aleph represented a glottal stop, a phoneme the Greeks didn't need, so they threw it away and invented vowel letters (which Semitic writing didn't need so much and didn't use).
So they made alpha a vowel letter. Beth became beta and /b/ is /b/, pretty much the same thing. Gimel /g/ became gamma /g/, and the letter still had the same camel-like hump.
When the Romans borrowed Greek letters, alpha became <A>, beta <B>, and gamma <C>. But it no longer meant /g/; it got devoiced to /k/; <C> always represents /k/, in Classical Latin (Medieval Latin is quite another matter).
And that's the last time that <C> always represents anything. When Latin split into the Romance languages, and Latin writing became a standard for other languages, <C> split into many varieties, depending on the original histories of the various languages, what sound changes had occurred when to which one, and which words had been borrowed into which languages (before or after the sound changes). Some of these variations acquired new spellings as <CH>, because the grapheme <H> is often used to differentiate letters.
From the standpoint of English, the various pronunciations of <CH> include:
At the risk of preaching to the choir, for many here have heard this tired old refrain before, I'm afraid that with this one, as with everything in English spelling, you need to chalk it all up to the overriding truism that in English there is no consistent connection between the spelling and the pronunciation, for no sooner than you think you find one, to your chagrin you also find an exception or three.
There is no other way than to check a dictionary each time: that is the only abiding rule that will do you any measurable good.
Here's a list of words with ch in them, some of which are rare, some of which are common, and many of which are cantanchorous:
Achaean, ache, achieve, Aeschylean, affiche, amuse-bouche, anchor, anchovy, anchusin, Anschluss, Appalachian, Archaea, archaeology, archaic, archipelago, architect, archive, bacchanal, Bach, Bacharach, Barchetta, bardache, bichon, blanche, borsch, brachiate, branchiate, brioche, brochure, bronchial, bronchitis, bruschetta, cache, cachet, cachou, cartouche, cha, chagrin, chai, chaise, chaitya, chalet, chameleon, champagne, chandelier, chandler, chaos, chaperone, character, charade, charisma, charlatan, Charlotte, chasm, château, chatelaine, chaton, chattel, Chaucer, chauffeur, chauvinist, Cheech, chef, cheiropod, chemical, chemise, chemist, Cher, Cheryl, Cheshire, Chevrolet, chevron, Cheyenne, chianti, chiaroscuro, chic, Chicago, chiffon, chinook, chirality, chiton, chivalry, chive, chlorine, choenix, choir, cholera, Chopin, choral, chord, choreography, chorus, choucroute, chough, choux, chowry, christen, chrome, chronic, chronological, chrysanthemum, chupatti, churl, chute, chutzpah, chyometer, cleruchial, cliché, coachee, coffee-klatsch, conche, corniche, couch, couché, couchette, crèche, croche, crochet, douche, echo, epoch, eschalot, eschatology, escheator, eunuch, Fischer, flèche, fuchsia, hypochondriac, ichneumon, ichor, ichthyology, ischaemia, Kampuchean, kerchief, kitsch, Lachmann, lachrymous, lechayim, leche, lecher, lechwe, lich, lichen, louche, macchiato, machete, Machiavelli, machine, machismo, Manichaean, Maraschino, marchioness, matriarch, mechanic, Michael, Michelle, Michigan, microfiche, milch, mischievous, monachal, monarchy, Münchausen, Munich, musichall, mustache, niche, Nietzschean, Noachian, nonchalance, ochre, orchestra, orchid, Pachycephalosaurus, pancheon, parachute, parcheesi, parochial, paschal, patriarch, petechia, pistachio, porch, Porche, psychic, putsch, quiche, ricochet, ruche, saccharine, saurischian, schedule, schefflera, Scheherazade, scheme, scherzo, schimmel, schism, schist, schitzy, schizophrenic, schizzo, Schlitz, schloch, schlong, schmaltz, schnapps, schnauzer, scholar, school, schooner, Schopenhauer, schryari, schwa, seneschal, stancheon, stomach, synchisite, synchronize, tcha, Tchaikovski, technician, technique, technology, tschermakite, Tsuchido, tunichood, Wirtschaftswunder, zucchini.
If you prefer to come at those "tail-sorted", that's:
Archaea, cha, tcha, petechia, ischaemia, fuchsia, charisma, schefflera, cholera, orchestra, Barchetta, bruschetta, schwa, chaitya, hypochondriac, archaic, chic, psychic, mechanic, schizophrenic, chronic, orchid, tunichood, cheiropod, chord, nonchalance, charade, Scheherazade, coachee, ache, cache, bardache, mustache, leche, affiche, crèche, flèche, microfiche, niche, cliché, corniche, quiche, blanche, conche, brioche, croche, Porche, amuse-bouche, douche, couché, louche, cartouche, ruche, Michelle, schedule, scheme, chrome, champagne, chatelaine, machine, saccharine, chlorine, Cheyenne, chaperone, ochre, Cheshire, brochure, chaise, chemise, brachiate, branchiate, machete, tschermakite, synchisite, couchette, Charlotte, chute, parachute, choucroute, technique, achieve, chive, archive, lechwe, synchronize, chef, kerchief, schlong, chutzpah, Bach, stomach, Bacharach, Cheech, lich, Munich, milch, schloch, epoch, matriarch, patriarch, porch, borsch, coffee-klatsch, kitsch, putsch, eunuch, couch, chough, chai, Tchaikovski, Machiavelli, zucchini, schryari, parcheesi, chianti, chupatti, chinook, chronological, chemical, monachal, paschal, seneschal, bronchial, parochial, cleruchial, bacchanal, choral, Michael, schimmel, chattel, musichall, school, churl, Cheryl, lechayim, chasm, schism, chrysanthemum, Achaean, Manichaean, Nietzschean, Kampuchean, Aeschylean, Michigan, technician, Appalachian, Noachian, saurischian, charlatan, lichen, christen, Münchausen, Chopin, chagrin, anchusin, Lachmann, pancheon, stancheon, chameleon, chiffon, bichon, ichneumon, chevron, chaton, chiton, Tsuchido, Chicago, archipelago, echo, pistachio, machismo, Maraschino, chiaroscuro, macchiato, scherzo, schizzo, scholar, Chaucer, Wirtschaftswunder, Cher, lecher, Fischer, chandelier, chandler, schooner, character, chyometer, Schopenhauer, schnauzer, choir, ichor, anchor, escheator, chauffeur, bronchitis, chaos, schnapps, marchioness, Anschluss, lachrymous, mischievous, chorus, Pachycephalosaurus, architect, cachet, ricochet, crochet, chalet, Chevrolet, eschalot, schist, chemist, chauvinist, cachou, château, choenix, choux, archaeology, technology, eschatology, ichthyology, monarchy, choreography, chivalry, chowry, chirality, anchovy, schitzy, Schlitz, schmaltz.
You'll note that I've included several words with multiple pronunciations, something that will get in the way of most of the simpler forms of rule-making.
As tchrist mentioned in his answer to a related question, Rules to pronounce "cha-" words, the digraph "ch" most commonly represents the sound /tʃ/, as in charm. It can also represent sounds like /ʃ/, /k/ and /x/, or no consonant sound at all as in yacht. In general, there is no way to tell which sound it represents, but there are a few ways you can rule out certain pronunciations in some cases.
A useful (if limited) rule, mentioned by tchrist, relies on the fact that the affricate sound /tʃ/ cannot occur before other consonants at the start of a word. (There are no words that start in /tʃl/ or //tʃr/ in English, unless you consider words like "train" to start with /tʃr/, and the initial consonant cluster of "train" is always written "tr" in any case.) So when "chl" or "chr" occur at the start of a word, they always represent /kl/ or /kr/. However, before a vowel, either /tʃ/ or /k/ is possible in English, so "cha" is ambiguous.
It may be somewhat useful to know that "ch" generally represents /k/ in words from Greek. Some spelling patterns are strongly associated with words from Greek, such as the digraph "ph", the use of "y" as a vowel in the middle of a word, or the use of "x" at the start of a word. So if these spelling patterns are present, it's likely that "ch" will represent /k/. Examples: chyme, choreography, xerarch. I found a few examples where this may provide a clue to the pronunciation of "cha": chalcography, chalybite, Charybdis.
In addition, there are certain sequences of letters that do not generally occur in words from Greek. The presence of these may be a clue that "ch" is not pronounced as /k/.
"ck": For example, if a word contains the digraph "ck", then "ch" will probably be pronounced /tʃ/. Here are some examples from the Onelook Dictionary Search: Words that match the pattern ch*ck*. However, this is not that useful for "cha" specifically, because the only word that happens to follow the pattern "cha*ck*" is the uncommon "charlock".
"f": Another letter that doesn't generally occur in words from Greek is "f". In most words containing "f" and "ch", the "ch" is pronounced as /tʃ/, as in "chief" or "cheer". However, there are some words where it is pronounced /ʃ/ instead, such as "chef". Some words with "cha" and "f" are chaff, chafe, chamfer.
