Lesson 41: If and when with と たら なら
と is used to link two events where one is a natural consequence of the other, in a strongly sequential way.
飲み続けると危ない薬 Medicine that's dangerous if you keep taking it.
たら is used to link two events where one sequentially comes after the other, but without literally stating that there's a direct causation between the two.
たら attaches to the reduced verb-sticky form.
私と話しててもつまらないんだったら私帰るわ
I'll go home if it's boring just talking to me.
(note: the doubled て comes from ている -> てる -> てて)
Because it doesn't state direct causation, it's very appropriate for questions that depend on something hypothetical.
来たらどうですか How about you come over?
なら is used when the speaker supposes the first statement is true, and that it implies that the second statement is true. There's basically no restriction on sequence or causation here, only the nature of the truth of the statements. In fact, the second statement can "happen" before the conditional statement does, which other condition constructions can't do.
Normally, なら only attaches to nouns. When you use なら with verbs and い-adjectives, it's normal to attach の as a nominalizer (see Lesson 31). However, putting の between the verb/い-adjective and なら is optional.
Using の can give the nuance that the condition is a instance of the statement, like being sick. Dropping の can give the nuance that the condition is a general thing that might not be true right now, like when it rains.
The meaning of なら does not change because of adding the の. The difference in nuance comes from the verb or い-adjective being nominalized, not の coming before なら. The exact meaning of the statement does not change much.
行くのなら If you're going
行くなら If you go
Sometimes なら is said as ならば. In fact, ならば is where なら comes from. This ば is the one we already know.