"Those trees are pretty."
Translation:Ty stromy jsou hezké.
If we are talking about nominative plural forms, then 'ti' is used for masculine animate and 'ty' for masculine inanimate nouns. 'Strom' is inanimate.
Ah, now I understand. However, I would never think that a tree is not a living thing.
Grammatical animateness does not follow physical living/non-living. Even completely non-living entities can be (grammatically) animate and there can be pairs of animate/inanimate words for the same thing.