Learn Turkish: Bu yıl tatil yapamadık. - We could not take a holiday this year.

Exactly! In Turkish, you 'do' a holiday (tatil yapmak) rather than 'take' one. You will see yapmak used in many common combinations where English uses different verbs, such as kahvaltı yapmak (to have breakfast) or banyo yapmak (to take a bath).

How does yapamadık mean 'we could not take'? Where is the 'could not' part?

The 'could not' meaning comes from the -ama- suffix. The verb root is yap- (do). To say you are unable to do something, you add -ama- (or -eme-, depending on vowel harmony) right after the root. So, yap-ama-dı-k breaks down as do + unable to + past tense + we.

I often mix up 'we didn't' and 'we couldn't'. How can I tell them apart here?

Look closely for the extra a before the negative m. Yapmadık (yap-ma-dık) means 'we didn't do it' (a simple negative fact). But yapamadık (yap-ama-dık) means 'we couldn't do it' (we lacked the ability or opportunity). That single vowel changes the meaning entirely.

Why is there no suffix on 'bu yıl' to mean 'IN this year'?