Oft verrät dir ein einziges Wort, welche Zeitform du brauchst:

Wenn du … siehst	Denk an …	Beispiel
every day, usually, always	simple present	Bidu sleeps on the sofa every day.
now, right now, at the moment	present continuous	Bidu is sleeping right now.
yesterday, last week, two days ago	simple past	We arrived five days ago.
while (die Szene)	past continuous	While we were walking, it started to rain.
when I was a child, back then	used to / simple past	I used to swim in the river.
tomorrow, next week, in two weeks	going to / will / present continuous	I*'m flying** home in two weeks.*

Die beiden größten Fallen für Deutschsprachige: gestern + Perfekt wird simple past (I saw him yesterday), und das deutsche Präsens für die Zukunft wird going to, will oder present continuous.