Translation guide
The English word "mouse" has two main meanings: the small rodent and the computer device. In Japanese, these are completely different words. The animal is ネズミ, while the computer device is マウス. Context makes the meaning clear.
animal
Refers to mice and rats in general. For a small mouse specifically, ハツカネズミ can be used.
computer device
Borrowed from English. Used for all computer mice.
Referring to the small mammal, often found in homes or fields.
I saw a mouse in the kitchen. · The cat caught a mouse. · Mice like cheese.
General term for mice and rats. In everyday conversation, it covers both. If you need to specify a small mouse, you can say ハツカネズミ, but ネズミ is usually sufficient.
A mouse appeared in the kitchen.
A mouse ate the cheese.
Specifically a small mouse (Mus musculus). Used in scientific or precise contexts, or when distinguishing from rats.
Mice are often used in experiments.
Specifically a brown rat or sewer rat. Not a mouse, but sometimes included under ネズミ. Useful if you need to distinguish.
Brown rats are big.
Referring to the electronic device used to control a cursor on a screen.
Click the mouse. · My mouse needs new batteries. · Use the mouse to select the file.
The standard loanword for a computer mouse. Used in all contexts.
I bought a new mouse.
The mouse isn't working.
Describing a shy, quiet, or meek person, similar to the English idiom.
He's a bit of a mouse. · She's as quiet as a mouse.
Literally 'quiet like a mouse'. Used to describe someone very timid or meek.
She's as quiet as a mouse.
A more casual way to say 'a person like a mouse', implying timidity.
He's such a mouse, isn't he?
In Japanese, ネズミ only refers to the animal. Using it for the computer device will cause confusion. Always use マウス for the computer mouse.
Japanese nouns don't change for plural. ネズミ can mean 'mouse' or 'mice' depending on context. If you need to specify one mouse, you can say 一匹のネズミ (いっぴきのねずみ).