when and while are conjunctions.
In some uses when and while can mean the same, but they can also have slightly different meanings. We use them to introduce subordinate clauses. We can use when and while to mean ‘during the time that’, to connect two events happening at the same time:
We often use them with the past continuous to refer to background events:
For example:
While he was working, he often listened to music.

As well as
The phrase "as well as" creates one of those situations where you may have to make a judgment call about comma usage. As a general rule, it doesn't need a comma before it unless it’s a part of a non-restrictive clause.
For example:
I like carrot cake as well as chocolate cake, but lemon cake is my favorite.

Rather
We use rather to give emphasis to an adjective or adverb. It has a similar meaning to quite when quite is used with grad able words. It is more formal than quite. We often use it to express something unexpected or surprising
For example:
No, I’m rather busy, in fact.

Whether
We can use whether to report indirect yes-no questions and questions with or. If is more common than whether
For example:
The teachers will be asked whether they would recommend the book to their classes.

Could
Could is the past tense of can, but it also has uses apart from that--and that is where the confusion lies. When could is used as the past tense of can, it refers to an ability that a person generally had in the past or to something that was generally possible in the past ("When I was younger, I could run for miles," or "It used to be you could buy lunch for a dollar.").Could is also used to refer to something that you wish to have or do but that is not possible ("If only we could be free of this tyrant!"). It’s also used to describe something that was possible but did not happen, used with the verb have ("We could have won if we had practiced harder."). Could can also express annoyance or another strong emotion ("He could have asked me if I needed help!" or "I could have died I was so embarrassed!"), but can isn’t used this way

Would
Would is a little easier to understand because it is not related to could or can, and its uses are more specific. Would is used to talk about a possible situation that has not happened or that you are imagining ("I would quit my job if I won the lottery."), and it is also used with have to describe a situation that could have happened but did not ("She would have eaten less if she had known there was going to be dessert.").
Like could, would is used to describe something that you think is likely to be true or likely to happen ("I think the meal would [=could] feed ten people." "He said he would help me with this set of exercises when he gets home.").
Would can also be used to ask polite questions ("Would you mind if I had another cup of tea?"), or to wish for something ("I wish she would write a book."). It has several distinct uses that can and could do not, however: it is used to give opinions ("I wouldn't do that if I were you," or "I would say that she is in her 30s. [=it is my opinion that she is between 30 and 39 years old]"); it is used to express a willingness to do something ("I would be happy to take her to the airport"); and it is used to express frustration with something that you believe a person always does ("Well, he would say that, wouldn't he? He always says that!").

Might
We use 'might' to suggest a small possibility of something. Often we read that 'might' suggests a smaller possibility that 'may', there is in fact little difference and 'might is more usual than 'may' in spoken English.
For example:
She might be at home by now but it's not sure at all.

Let
'Let' can mean 'allow' or 'give permission
For example
Our boss let us leave early.
Let's + infinitive is often used to make a suggestion
For example
Let's go to the cinema tonight

I hope it will helped
Bes t of Luck

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"Nothing is impossible, the word itself says 'I'm possible'!