Learn Turkish: Yaşlı ağaca güvenme! - Do not trust the old tree!

This is an example of consonant mutation! When the suffix -a is added to a word ending in ç like ağaç, the hard ç softens into a c to make it easier to pronounce. So, ağaç + -a becomes ağaca.

Why does the word for tree have the 'to' suffix (-a) if the English translation is just 'trust the tree'?

In Turkish, the verb güvenmek (to trust) requires the object to take the dative case (the direction suffix). You can think of it as giving your trust to something. That is why we say ağaca instead of just ağaç.

How do we know güvenme is a negative command?

The root of the verb is güven, which acts as a direct command for a single person. To make a command negative, you simply add the negative suffix -me (or -ma, depending on vowel harmony), giving us güvenme (do not trust).

I know yaş means 'age'. How does it become 'old' in this sentence?

The suffix -lı means 'with' or 'having'. When you add it to yaş (age), you create yaşlı, which literally means 'having age'. This is the standard word used for 'old' when referring to people, animals, and living things like trees.