Questions & Answers about Мы практически закончили.
Because the subject is мы (we), which is plural.
In the Russian past tense, verbs agree with the subject in number, and in the singular they also agree in gender:
- я закончил — I finished (masculine speaker)
- я закончила — I finished (feminine speaker)
- оно закончило — it finished
- мы закончили — we finished
So -ли is the normal plural past-tense ending.
The infinitive is закончить, meaning to finish.
So:
- закончить — to finish
- мы закончили — we finished / we have finished
This is the perfective verb, which matters a lot in Russian.