Learn Hardcore Latin: Merula in horto salit et cibum quaerit. - A blackbird hops in the garden and looks for food.

Because merula is in the nominative singular, the case normally used for the subject in Latin. It is the one doing the actions of salit and quaerit.

So in this sentence, merula is the thing that jumps and looks for food.

Why is there no word for the or a in the sentence?

Latin does not have articles like English the and a/an. A noun like merula can mean a blackbird or the blackbird, depending on the context.

The same is true for horto and cibum: they can be understood as in the garden / in a garden, food / the food, depending on the situation.

Why is it in horto and not in hortus?

Because the preposition in takes the ablative when it shows location.

in horto = in the garden
hortus is the nominative form, not the form needed after in here

So horto is the ablative singular of hortus.