There is a running joke in Muslim circles that inshallah means no. Will you make it on time? Inshallah. Translation: probably not. The joke is old enough to have escaped the community — non-Muslims now use it the same way. And the joke would be harmless except that the actual word is doing serious theological work, and the misuse has worn that work away.
In shāʼ Allāh. If Allah wills. Three Arabic words, one of which is a small conditional particle, one of which is a verb of divine choice, one of which is the proper name of God. The phrase is also a direct Qurʼānic command — there is a verse that orders you to say it before any future commitment. That is not background detail. That is the whole reason the phrase exists.
We covered this phrase briefly in the meaning of dhikr as one of the four short formulas every Muslim says. This essay is the full version — the grammar, the verse, and what the phrase is asking from you when you use it correctly.