Tenses

Never choose a tense by feel. Every question carries a time signal, and the signal decides the form.

1. Simple present vs present continuous. Simple present for habits and universal truths ("Water boils at 100°C"); continuous for what is happening now or is temporary. Stative verbs never take the continuous: know, believe, understand, like, hate, own, want, seem, contain. "I am knowing him" is always wrong.
2. Present perfect vs simple past. since, for, just, already, yet, so far, ever, never, recently → present perfect ("He has just left"). A finished time word — yesterday, last week, in 2019, ago, when I was young → simple past ("He left yesterday"). The two can never share a sentence.
3. Past perfect. Use it only for the earlier of two past actions, usually with before, after, by the time, when: "The train had left before we reached the station."
4. Future. will for a decision made now, be going to for a plan already made, present continuous for a fixed arrangement. In a time clause after when, until, before, after, as soon as, if, English uses the present, never "will": "When he comes, I will tell him."

The single most common mistake for a Marathi or Hindi speaker is writing since with a plain present tense: "I am living here since 2015", straight from "मी २०१५ पासून येथे राहत आहे". English marks a stretch that began in the past and still continues with the present perfect continuous: "I have been living here since 2015."

How to spot it: whenever you see since (a point in time) or for (a length of time), your verb must start with have / has / had — no exceptions.

since / for / just / already / yet / never → have-has-had (perfect) · yesterday / ago / last week / in 2019 → simple past · earlier of two past actions → had + V3 · after when/until/if → present, never will · stative verbs take no -ing

Fill in the blank: "He ____ in this office since 2018." Options: works / is working / has been working / worked. → has been working. "Since 2018" fixes a starting point that runs up to now, so the verb must be perfect, and because the work is still going on it is the perfect continuous. Sanity check: try the other options against the signal — "works" and "is working" have no link to 2018, and "worked" would need the job to be over, which would then demand "from 2018 to 2020" instead of "since".