Perfective:-
When an action has one beginning, a middle AND an end
Some of perfect verbs can be tricky here:
"Я полюбил..." can be interpreted as "I started to love...". "Вода потекла..." - "Water started to flow...".
You can think about it in two ways:
- there is no end of action. beginning only is important to us.
- perfectness is about this hidden "to start" verb, that is hidden in prefix "по-", "за-" and so on.
You should look in dictionaries to determine is perfect verb is about completition or about beginning of action.
for example: "Вода потекла по руслу" is about beginning. "Вода протекла по руслу" is about completition.