* 理由 (riyuu): This means "reason" or "grounds" in the sense of justification or motive. This is probably the most common translation if you're talking about the *reason* for something. For example, "on what grounds?" would be 何の理由で？ (nani no riyuu de?).
* 根拠 (konkyo): This means "basis" or "grounds" in the sense of evidence or foundation. This is suitable when referring to the factual basis for a claim or argument.
* 敷地 (shikiji): This means "grounds" or "site" referring to the land surrounding a building, such as the grounds of a school or a house.
* 土台 (dotai): This means "foundation" or "base," which can sometimes be used metaphorically as "grounds" for something.
To choose the best translation, you need to provide the context in which you're using the word "grounds."