Basically, your sentences are right. However, there some minor errors. Corrected below.

Quote Originally Posted by Antonio1986 View Post
1. It is impossible to remember which formula to use in order to solve this exercise = Невозможно вспомнить математическую формулу, с помощью которой вы можете решить эту задачу.
2. I remember the beautiful time we spend in England = Помню хорошее время, которое мы провели в Англии.
3. We remember my grandmother = Мы поминаем мою бабушку (we implied that she is dead)
1. Comma use (a comma between two clauses is mandatory); spelling error (с которой); "решить задачу с формулой" - grammatically maybe correct, but we do not say like that, we say "решить задачу с помощью формулы" or "решить задачу при помощи формулы". You can also say "решить задачу по формуле".

2. A relative pronoun between two clauses is mandatory. The English way of wording (I remember the beautiful time we spend in England), where "which" is omitted (compare: I remember the beautiful time which we spend in England) is impossible in Russian.

3. First: please note: "поминать бабушку" does not mean just "to remember a grandmother who is dead". It means a special ritual of "поминки" with special dishes (mandatory are pancakes with honey) and usually vodka. In Russian culture, "поминки" are held: on the day of funerals; then on 9-th day since one's death; then on 40-th day since one's death; then half a year; a year, and every year on the date of death. That's what it means.
Second: if you say "We remember my grandmother", then "my" is "мою". Your version "Мы поминаем свою бабушку" is understood as "our grandmother", since "свою" is related to "мы".