Polish verbs (like English) have aspects. One example of this in English is that incomplete or imperfect actions can be expressed using by adding "ing" and using to be as the modal, or auxiliary verb, e.g.:
- I am going
- He is fishing
- He is riding his bike
In all of those examples, I'm describing an action that is happening right now and is incomplete (yet to finish). It's sometimes called the continuous tense in English.
We also have perfect forms in English (just like Polish), these forms are used to express habitual or complete actions, e.g.:
- He fishes (it's his hobby)
- I go to school (this is something I do every day)
- He works at a garage (I'm telling you what his job is, not what he's necessarily doing right now)
That subtle difference in English imperfect and perfect forms , in Polish is expressed not by changing the verb ending, but by using the perfect form of the verb instead (e.g. jeść vs zjeść)
That's why it's not correct to translate "Ty i ja idziemy" (imperfect ) to "You and I walk" (perfect), the essence of the meaning is that you and I are walking (right now), not something we do on a casual or habitual basis.
There's a much more detailed post (and very useful) post about it here:
Well, but perfective verbs actually only come into the picture in the Past and Future Tenses... apart from "I am going"/"I go to school", there will be no difference between the tenses in your other examples. Only Verbs of Motion have a distinction in the Present Tense, and apart of them some verbs have a 'habitual' equivalent which is not obligatory though.
For example it is impossible to use "zjeść" in the Present Tense.
I was trying to illustrate the difference between perfect and imperfect aspects in English as opposed to presenting English sentences that would translate to Polish perfect equivalents.
The thing I'm trying to show is that perfect vs imperfect exists in English, and that most native speakers don't even realise they're already doing it.
If they can wrap their head around that, hopefully the leap to Polish won't be quite as hard.