as you can probably infer from the subject I sometimes don't know how to distinguish between "now" and "by now".

E.g.: Have you finished your meal now / by now?

I rarely have the same problems with "sure" and "for sure":

E.g.: Is this sure / for sure?

Although I'd presumebly chose the right phrase, I don't know why I'd do so...

Are there any general rules concerning these problems?

Thanks in advance!
Welcome to the Forums,

I think there is a subtle difference between the two sentences. If you ask someone " Have you finished your meal now?" you expected them to finish it in another time, in a sooner or later time rather than then. But if you ask "Have you finished your meal by now ?", you are asking about if considering all the circumstances (if he'd been home in time, if the food was ready, if he had enough time, ...) he has finished his food by then. Hope that it is clear. There are other helpful members who will give you more clarification if needed.
Hello Mister Pink

Just to supplement LL's comments:

Since 'now' means 'at this moment', and 'by now' means 'before this moment', neither is particularly idiomatic with 'have you finished your meal'.

You would be more likely to say e.g.

1. Have you finished your meal yet?

- if you don't know whether he has finished the meal.

2. Have you finished your meal already?

- if you're surprised that he has finished the meal so soon.

Students: We have free audio pronunciation exercises.
Thanks so far, but what about the sure- for sure- thing?

(It is really great that you get an answer to your problems so fast...)
Students: Are you brave enough to let our tutors analyse your pronunciation?
Hello Mister Pink

I think it would vary from idiom to idiom. For instance, if you take these two phrases:

1. This is real.

2. This is for real.

the first is a description of a thing or event, whereas the second implies an interpretation of a thing or event.

But with other examples, the relationship between the two phrases might be different. So I'm not sure you could extract a general rule.

Okay, thanks again!