Learn Croatian: Ptice vide veliko stablo. - The birds see a big tree.

Vidjeti is a regular -jeti verb. To form the present tense, we drop the -jeti to find the root, which is vid-. The present tense ending for the they form (3rd person plural) in this verb group is -e. Adding them together gives us vide.

Why is the word for birds ptice instead of ptica?

Ptica (bird) is a feminine noun ending in -a. When feminine nouns are the subject of the sentence (the nominative case) and there is more than one, the -a changes to -e.

The tree is the direct object being seen. Why do veliko and stablo still look like they are in the dictionary form instead of taking accusative case endings?

They actually are in the accusative case! Stablo (tree) is a neuter noun, and veliko (big) is an adjective matching it. A helpful rule for neuter nouns is that their accusative form (used for direct objects) is always exactly the same as their dictionary form. So, veliko stablo does not change its shape here.