〜なら
If it's…, then… (topic-based conditional)machine translation
FormationNoun + なら; plain-form verb / adjective + （の）なら
なら is a conditional that springs from a topic: you hear what the other person said (or see the situation) and respond with "if that's the case, then…". That's why なら is perfect for giving advice and suggestions: hearing that your friend is going to Kyoto, you say 京都へ行くなら、新幹線が便利ですよ.
A feature only なら has: the action in the second clause can happen BEFORE the first. 京都へ行くなら、新幹線がいい — you board the train first, then arrive in Kyoto. Say 京都へ行ったら and it's the reverse: you must reach Kyoto before doing the second clause. This is the fastest test for choosing between なら and たら.
With a noun, なら attaches directly and means "speaking of X": 寿司なら、あの店 (for sushi, that place is the best). This usage is almost like a topic-marking particle.
The other three patterns can't fill this role: と needs an inevitable cause-and-effect "whenever A, B"; ば is a neutral hypothetical "as long as A, then B"; and たら requires A to happen first, then B. When your meaning is "if it's case X (as you just said)", that's なら's home turf.
Examples
京都へ行くなら、新幹線が便利ですよ。
If you're going to Kyoto, the shinkansen is really convenient.
寿司なら、駅前の店が一番おいしいです。
If we're talking sushi, the place in front of the station is the best.
暇なら、ちょっと手伝ってください。
If you're free, please give me a hand for a moment.