- Context (kɒntɛkst)
- Control (kənˈtrəʊl)
The first one's pronunciation starts as kɒ whereas the second one is pronounced with the kə sound. Is there a way to infer the pronunciation of such words starting with "co"?
What factor decides the pronounce of words starting with “co” to be kə or kɒ?
That factor is stress or accent.
Context: There are two syllables in 'context'. The first syllable is stressed while the second one is not. When 'con' is stressed, it is usually pronounced as /kɒn/. In 'context', 'con' is stressed so it's pronounced as /kɒn/.
Control: There are two syllables in 'control'. The first syllable is unstressed while the second one is stressed. 'Con' is unstressed and it's pronounced as /kən/ so 'control' becomes /kənˈtrəʊl/.
/ə/ is called schwa, it (always) occurs in unstressed syllables. The vowel in unstressed syllables often reduces to schwa while the vowel in stressed syllable does not often reduce.
Details about schwa are available here
Well, how can I tell which syllables are stressed and which are not by just looking at spelling?
Peter Shor says in their comment:
You can't tell which syllables are stressed by just looking at the spelling; English spelling doesn't do that.
Dictionaries often have phonetic transcription of words.
- control -> /kənˈtrəʊl/.
- context -> /'kɒntɛkst/
- convert -> /kənˈvɜːt/
- contrast -> /'kɒn.trɑːst/
You'll notice that there's a diacritic mark (') in the transcription, the syllable following that mark is stressed/ accented.
Read Peter Shor's comment:
For words where the stress varies, like content and construct, the noun is accented on the first syllable and the verb or adjective is accented on the second syllable. And generally, for two-syllable words, nouns tend to be accented on the first syllable and verbs on the second syllable (although there are lots and lots of exceptions).
Note: this is not a 'rule'.
(You might want to read Stress and Vowel Reduction — Wikipedia)