Learn Hardcore Spanish (Spain): Ayer había llovido y la calle estaba mojada. - Yesterday it had rained and the street was wet.

The pluperfect había llovido presents the rain as happening earlier than another past reference point (yesterday, at the time you’re describing the street). It sets up a cause that precedes a later past state: it had rained earlier, and as a result the street was wet.

If you merely want to report the past event with no prior reference time, the default is the preterite: Ayer llovió.
Your sentence is fine as a cause–result narrative, and it’s even more natural with an explicit link: Ayer había llovido, así que la calle estaba mojada.

Is Ayer ha llovido correct in Spain?

No in standard Peninsular Spanish. With finished-time expressions like ayer, you use the preterite: Ayer llovió. The present perfect ha llovido is used for time frames that include the present (e.g., Hoy ha llovido, Esta mañana ha llovido). In much of Latin America, the preterite is common even for hoy.

Could I say Ayer estaba lloviendo instead? What’s the difference?

Ayer estaba lloviendo means it was raining at that moment yesterday (an ongoing action). Había llovido says the rain had already happened before the moment you’re describing. So:

Ayer estaba lloviendo y la calle estaba mojada = it was raining then.
Ayer había llovido y la calle estaba mojada = it had rained earlier; now the street is wet.

Why not hubo llovido?

Because modern Spanish doesn’t use the form hubo + participle (the pretérito anterior) in speech, and it’s especially unnatural with weather verbs. Use había llovido (pluperfect) or llovió (preterite).

Why is it estaba mojada with estar, not era mojada with ser?

Estar describes a temporary or resulting state. A street being wet is a condition that can change, so La calle estaba mojada is natural. La calle era mojada sounds like a permanent characteristic and is unidiomatic here. If you wanted a passive event, you could say La lluvia mojó la calle (the rain wetted the street), but not La calle fue mojada unless you’re forcing a passive with an agent.

Why is it mojada and not mojado?

Agreement. Calle is feminine singular (la calle), so the adjective/participial adjective must be feminine singular: mojada.

El suelo estaba mojado.
Las calles estaban mojadas.

Does the past participle agree after haber?

No. With auxiliary haber, the participle is invariable:

Había llovido (not ×había llovida).
Ella había cerrado la puerta (not ×había cerrada). Here, mojada agrees because it’s an adjective with estar, not part of haber + participle.

Could I rephrase as La lluvia había mojado la calle?

Yes. That makes the cause explicit and focuses on the action that produced the state. Slight nuance:

Había llovido y la calle estaba mojada highlights the prior event and the resulting state.
La lluvia había mojado la calle highlights the agent (the rain) doing the wetting.

Would La calle estuvo mojada work? How is it different from estaba?

Yes, but it changes the aspect. Estuvo (preterite) presents a completed, delimited state: it was wet for a specific, finished period.

La calle estuvo mojada toda la mañana.
La calle estaba mojada is descriptive/background, not bounded in time.

Can I change the word order to Había llovido ayer?

Yes. Both Ayer había llovido and Había llovido ayer are grammatical. Placing ayer first is a bit more common/natural because it sets the time frame up front.

How does llover conjugate here? Is it impersonal?

Yes, llover is impersonal in weather uses and takes only 3rd person singular:

Llueve, llovió, llovía, ha llovido, había llovido, va a llover, estaba lloviendo. Forms like ×llovieron (for weather) are incorrect in standard usage.

Is llovido irregular? Any pronunciation tips?

It’s regular: llover → llovido.
In most of Spain, ll sounds like y (yeísmo), so llovido ≈ [yo-βí-do]. The letter v is pronounced like a soft b in Spanish.

Could I use húmeda instead of mojada?

You can, but the nuance changes:

Mojada = clearly wet, with visible water.
Húmeda = damp/moist, less water than mojada. So choose based on how wet the street was.

Is the connector y the best choice? What about así que or porque?

All can work, with different relations:

… había llovido, y … neutral coordination.
… había llovido, así que la calle estaba mojada. explicit cause → result.
La calle estaba mojada porque había llovido. result → cause. Avoid reversing cause and effect unnaturally (e.g., Había llovido porque la calle estaba mojada suggests the wet street caused the rain).