Quote Originally Posted by Throbert McGee View Post
For English-speaking students of Russian, I've written a short dialogue for another language blog with usage examples for several of the "modal" Russian verbs discussed in this thread:

Maria, Carlos, and Lola

Mostly the verbs are in the present tense, because the primary emphasis is on the difference between positive and negated modal constructions, such as:

Тебе надо позвонить Карлосу.
You have to call Carlos.
Тебе не надо звонить Карлосу.
You shouldn't call Carlos.
Ты можешь не звонить Карлосу.
You don't have to call Carlos (if you'd rather not).
Ты не можешь звонить Карлосу.
You cannot call Carlos (because I threw your cellphone into the aquarium).

Thus, I didn't really get deeply into a discussion of tense or aspect.

I would repost the whole thing here, but technically it's work-for-hire under copyright to the other blog. Anyway, I hope it's a useful summary for students, and I would certainly welcome comments from native Russians if there are problems with the phrasings I use.
I'd say everything's correct except a slight nuance with "не надо" phrase.

I think "не надо" can have two shades of meaning, so it can be a bit ambiguous sometimes.

1. "не надо" can express "mild" advice not to do something: Тебе не надо звонить Карлосу. ~ I recommend to you not to call Carlos. Otherwise, you may run is some problems.
I think, that is what the English phrase "You shouldn't call Carlos" means (please correct me if I get it wrong).

2. "не надо" can express just the absence of necessity: Тебе не надо звонить Карлосу. ~ You do not need to call Carlos. You may do it, but that is not necessary.