How do you know when to use なら or たら?

Use なら when you accept the condition as a given context and respond to it, like そんなに高いのなら買えません. Use たら when the main clause happens after the condition is realized, like 先生に聞いたら分かった.

JLPT Grammar Comparison

なら vs たら

All comparisons

How to tell them apart

The core contrast is timing and certainty. なら takes the preceding clause as a supposition or established context and comments on it, so the main clause is usually a judgment, advice, or intention, not something that happens after. たら is sequential: the main clause happens after the たら clause is realized, so it works for

なら Use なら when you accept the condition as a given context and respond to it, like そんなに高いのなら買えません.
たら Use たら when the main clause happens after the condition is realized, like 先生に聞いたら分かった.

Grammar point	Meaning	JLPT level
なら nara	conditional 'if'	N4
たら tara	used for 'if' or 'when' conditions	N4

なら nara

conditional 'if'

In these examples なら appears in the pattern なければならない (also なくてはならない), which attaches to the negative ない form of a verb. It expresses that something must be done, i.e. the action is required or unavoidable.

明日あしたまでに書かかなければならないレポートがいくつかある。

I have a few essays to write by tomorrow.
明日までに仕事を終えてしまわなければならない。

I must have my work finished by tomorrow.

Full なら grammar page →

たら tara

used for 'if' or 'when' conditions

たら attaches to the past (ta) form of verbs and i-adjectives, and to nouns or na-adjectives with だった, to make a conditional meaning 'if' or 'when'. It works in both casual and polite speech, and can also appear in wishes or counterfactual statements.

よく聞きいたら、ネタじゃなくてマジボケだったみたいです。

When I asked afterwards it seems he hadn't said that as a joke.
何か私にできることがありましたらお知らせ下さい。

Let me know if there is anything I can do.

Full たら grammar page →

なら and たら are both JLPT N4 grammar points that Japanese learners often mix up. This page compares them side by side on meanings, usage and example sentences, so you can tell them apart.