Hi,

What does "all along" mean? What is the difference between "all along" and "always"? And in which case is it correct to use "all along"?

As a time expression, 'all along' means 'during the complete process/event. There's also a sense of mild rebuke in the phrase. Examples -

You watched me trying hard to solve this problem and you knew the solution all along. The implication is you should have told me, but you didn't.

Tom knew Mary for years. He loved her all along, but he only told her on the day he died. (Like Cyrano De Bergerac, if you perhaps know the famous story) Again, the implication is that he should have told her.

Best wishes, Clive

1.Does 'all along' always have a sense of mild rebke in any context or not necessary? Can we use it without a sense of mild rebuke?

Consider 'I was  ill for 6 months, and Tom took care of me all along'. There's no rebuke here, just a stress on the duration

2."You watched me trying hard to solve this problem and you knew the solution all along."

Can we use 'all this time', 'all the time', 'the whole/entire time' intead of 'all along' in your example above without changing the meaning?  More or less, yes.

...and you knew the solution all this time.
...and you knew the solution all the time.
...and your knew the solution the whole/entire time.

3.Do those expressions also have a sense of mild rebuke like 'all along'? In a context like the above, yes.

Clive

What do you mean by 'more or less' here? Is there any subtle difference among them?

"We talked on the road the whole time/all this time/all the time/all along when going there."
Are these expressions interchangeable here? Yes, but possible small differences may exist in certain contexts. Nonverbal things like tone and expression may also have an effect.

Here are some possible differences.

the whole time stresses there were no pauses

all this time shows we are speaking close to the time we are speaking about

all the time stresses 100% of the time

all along as discussed earlier

Clive

1.Is there any difference between 'the whole time' and 'the entire time' here? Not really. 'Whole' is perhaps more casual.

2."all this time shows we are speaking close to the time we are speaking about"
I'm sorry I can't understand your explanation about 'all this time' here. Could you describe the situation? It's because the use of the word 'this' suggests the time was close. If we were talking about something that happened 20 years ago, I'd say 'that time'.

3.'We talked on the road all along when going there.'

(1)Can I use 'on the road' if we go there on foot in the example? Yes.
(2)Can I say
'We talked on our way there all along.' Use the word order 'We talked all along our way'. But 'all along' now sounds like you are talking not about time but about the distance that you travelled. eg 'There was garbage lying all along the road'. 

instead of
'We talked on the road all along when going there.'Similar comment.

Clive