Learing English: How do you know whether to use "would" or "will"

I am an English teacher. There are 4 usages of WILL (1) We often use WILL in these situations (a) Offering to do something - That bag looks heavy. I'll help you with it (not "I help") (b) Agreeing to do something - You know that book I lent you. Can I have it back if you've finished with it? - Of course. I'll give it to you this afternoon. (not "I give") (c) Promising to do something - Thanks for lending me the money. I'll pay you back on Friday. (not "I give") - I won't tell anymore what happening. I promise. (d) Asking somebody to do something (Will you...?) - Will you please be uiet? I'm trying to concentrate. - Will you shut the door, please? Most common of all the WILL usages is :- We use I'll (= I will) when we decide to do something at the time of speaking. - Oh, I've left the door open. I'll go and shut it. - I feel a bit hungry. I think I'll have something to eat. - I don't think I'll go out tonight. I'm too tired. (2) WOULD (a) We use WOULD ('d) when we imagine a situation or action. - It would be nice to have a holiday but we can't afford it. - I'm not going to bed yet. I'm not tired and I wouldn't sleep. - I would stay a bit longer but I really have to go now. (b) sometimes we use WOULD in the past (of will/won't) - Tom said he'd phone me on Sunday. - Ann promised that she wouldn't be late. - Liz was angry because the car wouldn't start. (c) You can also use WOULD when you talk about things that happened regularly in the past. - when we were children, we lived by the sea. In summer, if the weather was fine, we would all get up early and go for a wim. ( = we did this regularly) - Whenever, I get angry, I'd (would) walk out of the room.

will means a definite plan. example: How long will it take to reach London? While would is hypothetical example: How long would it take to reach London? In other sentence, would is the past tense of will.

I'm not sure anymore, and I don't know how to explain. I'm also not the expert when it comes to grammar but as far as I remember, use "will" when pertaining to an action that you still are about to do. Example: I will go to the market. --- You still haven't done it. You WILL still do it. With "would", you use it when pertaining to an action that you were about to do in the past (and may have already done). Example: I told(past) Mom that I WOULD go to the market. It's like you pair them. present with present, and past with past. Would is just the past form of will. Basically that's it. But there are many ways of using "would". Hope this helped? And hope this is correct.