Present Simple (I/you/we/they = verb; he/she/it = verb + s)
1a - usually, always, never, sometimes, now and then, etc. (FACT/HABIT)
Present Continuous (am/is/are + verb + ing)
1b - these days, currently, for the time being, this week, this period etc. (BREAK FROM HABIT)
1c - always/never ! (IRRITATION)
Past Simple (regular verbs = verb + ed; irregular verbs: 2nd form -> study!!!)
2a - yesterday, ago, last night, in 1980 etc.. (ACTION FINISHED - WHEN = MENTIONED)
Present Perfect (regular verbs = verb + ed; irregular verbs: 3rd form -> study!!!)
2b -since, for, how long, ever, never, always, just, recently, already, yet, still, so far, up till now)
(ACTION STILL GOING ON/RECENT ACTION & WHEN NOT IMPORTANT OR MENTIONED / RESULT/EXPERIENCE)
3b - He was doing the dishes while she was watching TV. (TWO LONGER ACTIONS GOING ON AT THE SAME TIME)
4a - I took my bike, picked up my friend and then went to school.(SERIES OF EVENTS - 3 OR MORE - IN THE PAST)
4b - Before I went to school, I had already gone to the gym. (STH. HAPPENED BEFORE STH ELSE)