Which kind of 'if' — and how inevitable the result is
Each one below is our full write-up of that form, in the order worth reading them. Nothing is summarised — you are looking at the same pages the grammar index links to, put next to each other.
if ~, when ~
V-た / い-adj-かった / N・な-adj + だった + ら
The most flexible conditional, and the one to reach for when unsure. It covers both "if" and "once/when", and unlike と it allows a request, invitation or opinion in the second half.
Built from the past form, but it says nothing about the past: 時間があったら means "if there's time", not "there was time".
if ~ (then)
五段 -u → -eba · 一段 -る → -れば · い-adj -い → -ければ
A more formal, more logical "if" — it presents a general condition rather than a specific occasion. Common in advice and in set phrases: 〜ばいい (you should just ~), 〜ば〜ほど (the more ~, the more ~).
Avoid it when the second half is a command or request about the same subject; use たら there instead.
whenever ~, if ~ (always)
V-plain / い-adj / N・な-adj + だ + と
For results that always follow — natural laws, machines, directions, habits. "Do A and B inevitably happens."
The restriction is the exam point: the second half cannot be a request, invitation, permission or command. 春になると暖かくなります is fine; ×春になると出かけてください is not.
if it's the case that ~, if you mean ~
N / な-adj / V-plain + なら
Picks up something the other person just said and responds to it. 京都へ行くなら、バスが便利ですよ — "if you're going to Kyoto (as you say), the bus is handy."
Unlike たら, the なら clause can happen *after* the main clause: you give the advice before they travel.