The conditional and provisional can be constructed in several ways. The bad news is that it can be tricky to choose which one to use. The good news is that even the less correct choices should be understandable. Here we'll try to explain the differences among these to give you an idea.
The main forms for "if/when A then B" are
|A と B|
Let's now look at each of these in turn.
The provisional and conditional forms are like the 〜て form in that they connect two sentences together. The second sentence is, however, not necessary if its meaning is contextually implied.
In cases describing a natural consequence one can use "と". This is similar to English: "Eat this apple and you'll be sick." which in Japanese would be "このリンゴを食べると病気になります。"
Verbs take on the provisional form by changing the final syllable from the う-row to the え-row and adding "ば". This applies to both consonant stem verbs, e.g. 行く -> 行けば, and vowel stem verbs, e.g. 食べる -> 食べれば, and 寝る -> 寝れば.
Nouns and adjectival nouns take on the provisional form by using the provisional form of the copula, "なら": "勉強なら習う", "if you study, you learn".
The conditional form is formed by adding "ら" to the plain past tense (ending in 〜た), e.g. 行ったら, 食べたら, and 寝たら.
Nouns and adjectival nouns take the conditional form of the copula, either the polite "でしたら" or the plain "だったら".
To use adjectives in a conditional expression, replace the final "い" with "ければ".
The word "もし" shows that the condition/provision is not necessarily true.
- もし、かなしかったら、わらおう！ // If [you] are sad, smile!
Sometimes it takes the form もしも, which also means if.