I know there is no strict rule on pronunciation of words in English but here my question is about the words which begin with 'con' and 'com', more than asking general rule.
When I look at the words begin with 'con' and 'com', I see mostly two patterns : One of them is schwa /ə/ sound and the other one is /o/ or /ɒ/ sound.But the second pronunciation can be pronounced slightly differently depending on dialect.
An example word for the first pronunciation is the word computer whose pronunciation is represented /kəmˈpjuː.tər/ in Cambridge Online Dictionaries.
An example for the second type of pronunciation for "co" could be the word contrast whose pronunciation is represented /ˈkɒn.trɑːst/ in Cambridge Online Dictionaries.
So my question is there any rule/pattern which can help us guess how to pronounce these words correctly?
I am not asking about the words like cool, come or coke. My question is about words like contract, combination, continue, combat.
Best Answer
A very reliable rule of thumb for British English and General American is this:
If the syllable con is stressed it will take a full strong, vowel, and the first syllable will be: /kɒn/.
If the syllable is not stressed we will find a schwa, /ə/, or no vowel at all in the first syllable: either /kn/ or /kən/.
However, we should bear in mind that some regional varieties of English will have a full vowel in the first syllable of many of these words regardless of stress. So in Yorkshire English, for example, many speakers will say /kɒnˈstɪtʃʊənt/.
This morpheme 'con' occurs a lot in English. That last sound in 'con', /n/, is very unstable both because it is an alveolar sound and also because it is nasal. It tends to change according to the following consonant. We find this prefix with different final consonants in words like: collect, commemorate, correlate. These types of word also show the same variation in their first syllables depending on the stress:
Both words collection and corroborate could be said with no schwa at all, and just a syllabic consonant - but the transcription for this is tricky in terms of syllabification, so I have taken the easy route and used a schwa. Similarly, computer could equally be said with a schwa instead of a syllabic /m/.
[Transcription note: in line with the Original Poster's question I have used Southern Standard British English transcriptions].