〜なら

なら is the topic-presupposing conditional — "given that X is the case, [then] Y." The X clause is information the speaker is conceding or hypothetically accepting, often because the listener just raised it. 寿司なら、銀座がいい — "If [you're talking about] sushi, Ginza is good."

The contrast with the other three conditionals is sharpest when the speaker is responding to a topic the listener just introduced. "I'm thinking of going to a restaurant" → 寿司なら、銀座がいい (acknowledging the topic of restaurants/sushi, then giving a recommendation). With たら or ば it would sound like the speaker is hypothesizing about the world rather than responding to the listener's frame.

なら attaches directly to nouns and na-adjectives (no copula needed: 学生なら, not 学生だなら). For verbs and i-adjectives it attaches to the plain form: 行くなら、早めに — "If you're going, [go] early."

Time order is also distinctive: with なら, the Y clause can describe an action that happens before the X condition would arise — 行くなら、地図を持って — "If you're [going to] go, take a map" (the map-taking happens before the going). The other conditionals can't reverse the temporal order like this.