Sacude el felpudo antes de entrar, que hoy ha llovido y todo está mojado.

Yes. Sacude is the affirmative tú command of sacudir. The speaker is giving an informal instruction to one person.

Spanish usually leaves out the subject pronoun, so (tú) sacude el felpudo means shake the doormat.

Related forms:

• sacuda = formal singular command
• sacudid = vosotros command
• sacudan = ustedes command

Because Spanish uses antes de + infinitive when the subject is the same as in the main clause.

Here, the person who should shake the doormat is also the person who will enter, so antes de entrar is the normal structure.

If the subject were different, Spanish would use antes de que + subjunctive, for example:

• Hazlo antes de que entre = Do it before he/she comes in.

Here que introduces the reason or explanation for the command.

In this kind of sentence, especially in everyday Spanish from Spain, que often works like because or since:

• Sacude el felpudo..., que hoy ha llovido...
• Shake the doormat..., since it has rained today...

It is a very common conversational pattern after orders, suggestions, or warnings.

Yes. Porque would be grammatically correct:

Sacude el felpudo antes de entrar porque hoy ha llovido y todo está mojado.

That sounds a bit more neutral and explicit. Using que sounds more natural in casual spoken Spanish, especially in Spain.

So:

• que = more conversational, very common after commands
• porque = more standard and direct

In Spain, the present perfect is very often used for actions connected to a time period that is still considered current, such as:

• hoy
• esta semana
• este año

So hoy ha llovido is very natural in Peninsular Spanish.

In much of Latin America, many speakers would be more likely to say hoy llovió instead. Both can be correct, but the preference is regional.

Because mojado describes a temporary condition: things are wet right now because it has rained.

Spanish usually uses estar for temporary states and conditions:

• está mojado = it is wet

Using ser here would sound wrong, because ser is not normally used for this kind of temporary physical state.

Because todo here means everything, and everything is treated as singular.

That is why the sentence has:

• todo
• está
• mojado

All three are singular.

If you were talking about several specific things, you would use plural forms, for example:

• Todas las cosas están mojadas = All the things are wet

But todo está mojado is the normal way to say everything is wet.

Spanish often omits subject pronouns when they are already clear from the verb or from the context.

• Sacude already tells you the subject is tú
• Ha llovido is an impersonal weather expression, so Spanish does not need a dummy subject like English it

So English says:

• It has rained

But Spanish simply says:

• Ha llovido

Yes, it sounds very much like Spain Spanish.

Two main clues are:

• hoy ha llovido, which is very typical in Peninsular Spanish
• que used after a command to give the reason, which is also very common in everyday speech in Spain

Speakers in other Spanish-speaking regions would still understand it, but they might naturally choose slightly different wording.