Learn Turkish: Sonbaharda hava serin. - In autumn the weather is cool.

You would attach the past tense suffix directly to the adjective. Because serin ends in a consonant and its last vowel is an 'i', it takes the suffix -di following vowel harmony rules. The sentence would simply become Sonbaharda hava serindi.

Where is the word for 'is' in this sentence?

In Turkish, simple present tense statements usually don't need a visible word for 'is' (like 'am/is/are' in English). Just saying hava serin (literally 'weather cool') is the completely natural and correct way to say 'the weather is cool'.

Is there a literal translation for the word sonbahar?

Yes! Son means 'last' or 'end', and bahar means 'spring'. So sonbahar literally translates to 'last spring', which is the Turkish word for autumn or fall. For contrast, the season of spring itself is often called ilkbahar (first spring).

Why is there a -da at the end of sonbahar?

The suffix -da is the locative case, which tells you where or when something happens ('in', 'on', or 'at'). By adding it to sonbahar (autumn), you get sonbaharda, which means 'in autumn'.