So if пойдем is used for "Let's go to the park" "пойдем в парк" when is пошли used?
I don't know if I translated that right.
Basically in this context (when you're making a suggestion) there's no difference.Пойдём can just be considered as a bit more persistent offer)
1. The first one means absolutely the same if you use them in the meaning : let's go. Off we go.
You just say: Пошли! Or Пойдем!
2. In the second case, there is a difference as these two verbs are used in different tenses.
•Пошли is used in the past and sometimes in the present: мы пошли в парк. We went to the park/we've just gone to the park/we're going to the park (we've moved/started to go already in the direction to the park)
•Пойдем is used as a future tense: Мы пойдем в парк.
We will go to the park/ Tomorrow we are going to the park
Мы пойдем в парк через час. - We'll go to the Park in an hour. future time
Они пошли в парк утром. - They went to the Park in the morning. past time
"Let's go to the park" = "Пойдем в парк". future time
"Пошли" используется в отношении уже совершенного действия.
"Пойдем" используется в отношении действия, которое планируете совершить.