Learn Arabic (Modern Standard): هرب الفأر تحت السرير.

Arabic sentences usually follow a Verb-Subject-Object (VSO) order. While you could technically say الفأر هرب, starting with the verb is the most natural and common way to express a narrative action like this in Modern Standard Arabic.

How does the word تحت (under) affect the grammar of the word السرير (the bed)?

Adverbs of location like تحت act as the first part of a possessive construction (iDafa). Because of this rule, the noun following it is forced into the genitive case. If you were pronouncing the full case endings, it would sound like 'taHta as-sariiri' with an 'i' sound at the end of the bed.

Why is the hamza in الفأر (the mouse) written on an alif (أ) instead of a waw or yaa?

The 'seat' of a middle hamza is decided by comparing its own vowel with the vowel of the letter right before it. The letter faa has a fatha (short 'a' sound) and the hamza itself has no vowel (sukun). Because the fatha is stronger than a sukun, the hamza sits on an alif.

Can هرب also mean 'to run away'?

Yes! هرب is the standard verb for fleeing, escaping, or running away. Whether the mouse broke out of a cage (escaped) or just scurried out of sight (ran away), this is the correct verb to use in this context.