I have realized that to pronounce the plural form of words ending in -th, we have to drop the letter "h". e.g. Months is pronounced /mʌnts/. But I have never seen this point mentioned in any context. I just want to make sure that this rule is correct and can be always applied.
The standard pronunciation of a word such as months is /mʌnθs/, as Barrie indicated. Here the /θ/ symbol indicates the th sound as found at the beginning of thin or the end of month.
However, you've correctly noticed that the word months is not often pronounced that way, and many people do indeed say something like /mʌnts/, using /t/ rather than /θ/ for the second-to-last sound. There are two things going on here:
- The sequence of sounds /θs/ is very difficult to say, and it is very often reduced to simple /s/, especially in rapid speech.
- The sequence /ns/ is also somewhat difficult to say, as the transition from nasal /n/ to voiceless spirant /s/ is problematic. The letter s following n is usually pronounced as /z/; where we have an actual /s/ sound following /n/, a /t/ often gets inserted in pronunciation. For example, the word chance often sounds just like the word chants. This intrusive /t/ is called an epenthetic consonant.
So I believe that what's really going on here is a two-stage process:
θs > s (deletion) ns > nts (epenthesis)
Both halves of this process can be frequently observed in a variety of words, while the direct change from /θ/ > /t/ is not widely attested.
This does not mean that the pronunciation /mʌnts/ is incorrect. It is, in fact, one of the most common spoken forms of this word, and it occurs because of normal, unobjectionable phonological processes in spoken English. You absolutely should not feel like there's anything wrong with months as munts.
Note that in any case it's very misleading to talk about "dropping the h". There is no /h/ sound anywhere in either pronunciation. The spelling convention th is just an orthographic convenience, and the th sound /θ/ has nothing to do with /h/.
Usually, when people pronounce the word "months" there is a process of th cluster simplification. The unvoiced 'TH' sound, /θ/, gets dropped.
- /mʌnθs/ ----> /mʌns/
However, if you want to make the sound for this word so that it's very native speaker-like, then you need to be careful to make the /n/ sound using your tongue behind your front top teeth. It won't sound so good if you make the sound with your tongue on the alveolar ridge (that's the little shelf behind your top teeth, where native English speakers pronounce /n/).
Occasionally, you might hear a native speaker accidentally introduce a little /t/ sound between the /n/ and the /s/. You don't need to worry about this. It just sometimes happens when speakers switch from /n/ to /s/ in normal speech.
Singular nouns in English that already end in unvoiced fricatives usually form plurals by adding voicing the fricative and then /z/ to then end. This is not constant across all words and speakers, but is a general trend left over from ancient forms of the language. Voicing the fricative also makes any ‘Canadian rising’ disappear, and the vowel tends to lengthen a bit compared with the singular form (not always so marked below).
- house /hʌus/ > houses /ˈhaʊzəz/
- elf /ɛlf/ > elves /ɛlvz/
- wolf /wʊlf/ > wolves /wʊlvz/
- wife /wʌif/ > wives /waɪvz/
- calf /kæf/ > calves /kævz/
- leaf /li(ː)f/ > leaves /liːvz/
- scarf /skʌɹf/ > scarves /skɑɹvz/
- bath /bæθ/ > baths /bæðz/
- moth /mɔθ/ > moths /mɔðz/
- birth /bɝθ/ > births /bɝðz/ (I don’t know how to rightly show the height difference in those two vowels; should it be /bɚðz/ for the second? Somehow, I don’t think so. But the first one is a higher vowel. Compare pert with bird, for example.)
- mouth /mʌuθ/ > mouths /maʊðz/
- booth /buθ/ > booths /buːðz/
- scythe /sʌiθ/ > scythes /saɪðz/
- path /pæθ/ > paths /pæðz/
- sheath /ʃi(ː)θ/ > sheaths /ʃiːðz/
- cloth /klɔθ/ > (originally) clothes /kloʊðz/
- oath /o(ʊ)θ/ > oaths /oʊðz/
- wreath /ɹi(ː)θ/ > wreaths /ɹiːðz/
- youth /ju(ː)θ/ > youths /juːðz/
The same thing occurs when forming the third-person singular of present tense verbs whose base forms end in fricatives. However, verbs are much more likely to be already voiced at the end anyway, as in the unvoiced endings of house, bath, cloth, half, and wolf as nouns, contrasting with the voiced endings of house, bathe, clothe, halve, and wolve as verbs.
However, in words like fifth, sixth, ninth, twelfth, and month, this doesn’t occur, so month /mʌnθ/ becomes months /mʌnθs/ or even /mʌns/, depending on the care of the speaker. I would say that the /θ/ is normally dropped in all but the most exacting of speech. The same sort of thing also tends to occur with the others I just mentioned, so fifths might be any of /fɪfθs/, /fɪθs/, or /fɪs/.
Those complex consonant clusters are all pretty much guaranteed to cause conniptions in non-native speakers whose mother tongues don’t include /θ/ — and sometimes even if they do. Both sixths and twelfths have four consonsant sounds in a row. This can be a nightmare if one’s native language doesn’t have liquid s’s, such as Spanish or Portuguese.