GrammarJLPT N5
→ if
If / In case (conditional emphasis adverb)
JLPT N5AdverbNeutral
もし is an adverb that appears at or near the beginning of a conditional clause to explicitly signal that the speaker is about to present a hypothetical situation. It does not create the conditional by itself — it always works together with a conditional ending such as ～たら, ～ば, ～なら, or ～と. Its role is to make the hypothetical nature of the statement unmistakably clear, much like the English word "if" placed at the very front of a sentence for emphasis. Without もし, the conditional forms still function perfectly well, so もし is technically optional; however, adding it draws the listener's attention and often implies that the scenario is less likely or purely imaginary. It is used across all registers — casual, neutral, and formal — and is one of the first conditional-related words learners encounter. A stronger variant, もしも, adds even more emphasis and is sometimes used in more dramatic or literary contexts, while 万が一（まんがいち） is used for very unlikely scenarios.
Examples
Example #1
もし明日雨が降ったら、家にいます。
Here もし is placed at the very beginning of the sentence to clearly mark what follows as a hypothetical condition. The actual conditional grammar is carried by ～たら, while もし serves as an advance signal that alerts the listener: "I'm about to describe a scenario that may or may not happen." Without もし, the sentence would still be grammatically correct, but adding it makes the hypothetical tone more explicit and natural-sounding, especially when beginning a conversation about future plans.If it rains tomorrow, I will stay home.
Structures
- General pattern
- もし + [clause] + conditional ending (～たら / ～ば / ～なら / ～と), + [result clause]
Common mistakes
A very common mistake is using もし alone without any conditional ending, producing sentences like "もし雨です、家にいます," which is ungrammatical because もし is not a conjunction and cannot create a conditional clause by itself — it must always be paired with ～たら, ～ば, ～なら, or ～と. Another frequent error is placing もし in the result clause rather than the condition clause; it should appear before the "if" part, not the "then" part. Learners also sometimes overuse もし in every conditional sentence, which can sound unnatural in cases where the condition is highly expected or routine, such as natural consequences (e.g., "水を飲めば、のどがかわかない" does not need もし because the outcome is a given). Finally, some learners confuse もし with もしかしたら ("perhaps / maybe"), which expresses speculation about reality rather than setting up a conditional clause.
Related
～たら～ば～なら～と (conditional)もしも万が一