mouse
The German word for mouse is 'Maus'. It is a noun and is used in the same context as in English, referring to a small rodent. In German, nouns are capitalized, so 'Maus' is always written with a capital 'M'.
The mouse is small.
This sentence is simply describing the size of the mouse, stating that it is small. In German, the definite article 'die' is used for feminine nouns, 'Maus' being one of them.
I have seen a mouse.
This sentence describes a past action where the speaker saw a mouse. In German, to form the perfect tense, 'haben' (have) is used as an auxiliary verb, followed by the past participle of the main verb, 'gesehen' (seen).
The cat chases the mouse.
This sentence describes an action where a cat is chasing a mouse. In German, the verb (in this case, 'jagt') typically comes in the second position in the sentence.