Ok, see this in the dictionary:
Why do we have to put an extra /g/ in front of /ər/?
But sing → /sɪŋ/ & singer → /ˈsɪŋər/ do not adhere to that rule. But "sing" is not an adjective.
Is it a rule that whenever we see /ŋ/ at the end of an adjective & if we want to put -er at the end of that word, then we will pronounce it like /ŋɡər/?
In both Southern Standard British English and General American, there is indeed a phonological generalisation that can be made such that adjectives ending in /ŋ/ have comparative forms ending in /gə/ (or /gər/ in Gen Am).
The phoneme /ŋ/ in English is phonologically interesting in its own right. For a start in English there are no words that begin with this morpheme. There are lots of words beginning with the other nasals /m/ and /n/, but none with /ŋ/. This makes /ŋ/ the only consonant in English which doesn't occur at the beginning of English words. Even /ʒ/ occurs at the beginning of one relatively frequent English word, the word genre.
Secondly the distribution of /ŋ/ within words is very interesting too. Here is an excerpt from Peter Roach's English Phonetics and Phonology: A Practical Course (2009, pp 46-48), which explains why and also addresses the adjective question:
Medially, ŋ occurs quite frequently, but there is in the BBC accent a rather complex and quite interesting rule concerning the question of when ŋ may be pronounced without a following plosive. When we find the letters 'nk' in the middle of a word in its orthographic form, a k will always be pronounced; however, some words with orthographic 'ng' in the middle will have a pronunciation containing ŋg and others will have ŋ without g. For example, in BBC pronunciation we find the following:
A: 'finger' fɪŋgə, 'anger' æŋgə
B: 'singer' sɪŋə, 'hanger' hæŋə
In the words of [...] A the ŋ is followed by g, while the words of [...] B have no g. What is the difference between A and B? The important difference is in the way the words are constructed - their morphology. The words of column B can be divided into two grammatical pieces: 'sing' + '-er', 'hang' + '- er'. These pieces are called morphemes, and we say that column B words are morphologically different from column A words, since these cannot be divided into two morphemes. 'Finger' and 'anger' consist of just one morpheme each.
We can summarise the position so far by saying that (within a word containing the letters 'ng' in the spelling) ŋ occurs without a following g if it occurs at the end of a morpheme; if it occurs in the middle of a morpheme it has a following g.
Let us now look at the ends of words ending orthographically with 'ng'. We find that these always end with ŋ; this ŋ is never followed by a g. Thus we find that the words 'sing' and 'hang' are pronounced as sɪŋ and hæŋ; to give a few more examples, 'song' is sɒŋ, 'bang' is bæŋ and 'long' is lɒŋ. We do not need a separate explanation for this: the rule given above, that no g is pronounced after ŋ at the end of a morpheme, works in these cases too, since the end of a word must also be the end of a morpheme. (If this point seems difficult, think of the comparable case of sentences and words: a sound or letter that comes at the end of a sentence must necessarily also come at the end of a word, so that the final k of the sentence 'This is a book' is also the final k of the word 'book'.)
Unfortunately, rules often have exceptions. The main exception to the above morpheme-based rule concerns the comparative and superlative suffixes '-er' and '-est'. According to the rule given above, the adjective 'long' will be pronounced lɒŋ, which is correct. It would also predict correctly that if we add another morpheme to 'long', such as the suffix '-ish', the pronunciation of ŋ would again be without a following g. However, it would additionally predict that the comparative and superlative forms 'longer' and 'longest' would be pronounced with no g following the ŋ, while in fact the correct pronunciation of the words is:
- 'longer' lɒŋgə 'longest' lɒŋgɪst
As a result of this, the rule must be modified: it must state that comparative and superlative forms of adjectives are to be treated as single-morpheme words for the purposes of this rule. It is important to remember that English speakers in general (apart from those trained in phonetics) are quite ignorant of this rule, and yet if a foreigner uses the wrong pronunciation (i.e. pronounces ŋg where ŋ should occur, or ŋ where ŋg should be used), they notice that a mispronunciation has occurred.
[...]
The velar nasal consonant ŋ is, in summary, phonetically simple (it is no more difficult to produce than m or n) but phonologically complex (it is, as we have seen, not easy to describe the contexts in which it occurs).
(pp. 46-48)
Richard Venezky summarises the situation in relation to adjectives in The American Way of Spelling (1999):
However, the pronunciation of any form ending in < nger > or < ngest > cannot be predicted unless the morphemic identities or and are known. If these are the comparative and superlative endings, then < ng > is pronounced /ŋɡ/, as in stronger, in most other cases the /ŋɡ/ cluster is leveled to /ŋ/, just as it is in word-final position. (p. 139)
We can demonstrate that this is because of the fact that these words are adjectives and not for another reason by comparing homophonous noun and adjective pairs.
Imagine that a cruise liner sinks in the middle of the ocean. All the inhabitants survive and end up on a desert island. After a couple of years they have built a new rudimentary society. The island however, is divided into two camps. There are those who are very happy to be away from their old lives and who don't want to be discovered or rescued. Then there are those that long to go back home and join their families who think the islanders should try and make contact with the outside world. The ones that long to go home are dubbed the longers. Notice that this word has no /g/ sound in it. The base of this word longer, is the verb long. The noun is made from adding the ER suffix (or, more strictly speaking the -ə(r) suffix) to this base. We can contrast this /g/-less word with the adjective longer meaning more long. This is made similarly by adding an -ə(r) to the base long. However, because this is an adjective form adding the suffix requires a /g/ to be inserted at the end of the root.
So, if the Longers were to have a change of heart, we could have the following sentence:
Note that this will not be true for all varieties of English. Some varieties always have a /g/ following an /ŋ/, regardless of the part of speech. Others may have no /g/ in the comparative adjective forms. Some may have other complicating factors determining whether or not a /g/ is present.
"Stronger" results from a morphological process which adds an ending to the stem "strong". "Singer results from a syntactic process which adds an ending to the word "sing". "G" is lost at the end of a word after velar nasal, but this does not happen at the end of a stem which is not also the end of a word.
In standard generative phonology (e.g., SPE), the difference is annotated by putting the # boundary before and after every word, so that "sing" is preceded and followed by #, but the "-er" agent suffix is not a word, so the combination is written "#sing#er#". However "strong+er" has only the weaker + morpheme boundary at the end of the stem "strong".
Aside from its effect in causing the deletion of preceding "g" after engma, the # boundary also determines the end of a stress word. The addition of "#er" to a verb never affects the position of the stresses in the verb.
/sɪŋɡər/.