Quando piove, l’erba è bagnata.

Yes. In Quando piove, l’erba è bagnata, quando introduces a time clause: when it rains.

It is very commonly followed by a full clause with a verb, as in:

• Quando arrivo, ti chiamo. = When I arrive, I’ll call you.
• Quando studi, impari. = When you study, you learn.

It can also be used as a question word:

• Quando parti? = When are you leaving?

So here, quando is a conjunction meaning when.

Because piovere (to rain) is usually used impersonally in Italian.

So Italian normally just says:

• Piove. = It’s raining.

There is no subject pronoun like English it. The verb appears in the third person singular form, but the subject is not expressed.

This is very common with weather verbs:

• Nevica. = It’s snowing.
• Grandina. = It’s hailing.

So piove simply means it rains / it’s raining, depending on context.

Because erba is a feminine singular noun, and the feminine singular definite article la changes to l’ before a vowel.

So:

• la casa = the house
• la strada = the street
• l’erba = the grass

This is called elision. The a of la drops because the next word begins with a vowel.

In Italian, noun gender is something you often have to learn together with the word. Erba is a feminine noun, so it takes feminine articles and adjectives:

• l’erba
• l’erba verde
• l’erba bagnata

A useful clue is that many nouns ending in -a are feminine, though not all. Here, erba is indeed feminine.

È is the verb essere (to be) in the third person singular: is.

The accent is important because:

• è = is
• e = and

So:

• l’erba è bagnata = the grass is wet
• pane e formaggio = bread and cheese

In writing, the accent helps distinguish these two very common words.

Because adjectives in Italian usually agree with the noun they describe in gender and number.

Here:

• erba is feminine singular
• so the adjective must also be feminine singular
• therefore: bagnata

Compare:

• il prato è bagnato = the lawn is wet
  (prato is masculine singular)
• l’erba è bagnata = the grass is wet
  (erba is feminine singular)
• le piante sono bagnate = the plants are wet
  (feminine plural)

Here, bagnata is an adjective, meaning wet.

The structure is:

• l’erba = the subject
• è = is
• bagnata = an adjective describing the subject

So this is a normal copular structure: subject + to be + adjective.

It is not a passive form here. It simply describes a state: the grass is wet.

The comma separates the introductory time clause from the main clause:

• Quando piove, l’erba è bagnata.

This is very natural in Italian, just as in English:

• When it rains, the grass is wet.

If the main clause comes first, the comma is often not needed:

• L’erba è bagnata quando piove.

Both word orders are possible, but the version with quando first often uses a comma.

Yes. That is also correct.

The difference is mostly one of focus and style:

• Quando piove, l’erba è bagnata.
  Starts with the condition/time setting: When it rains...
• L’erba è bagnata quando piove.
  Starts with the main idea: The grass is wet...

Both are grammatical and natural.

Yes, piove is present tense.

Italian present tense often covers meanings that English splits between:

• it rains
• it is raining
• sometimes even more general present uses depending on context

So quando piove can mean:

• when it rains (general/habitual)
• when it’s raining (more situational)

In this sentence, the general meaning when it rains is the most natural.

A simple pronunciation guide is:

• Quando ≈ KWAN-doh
• piove ≈ PYOH-veh
• l’erba ≈ LEHR-bah
• è ≈ eh
• bagnata ≈ ban-YAH-tah

A couple of useful details:

• pi in piove sounds like pyo
• gn in bagnata is like the ny sound in canyon

So bagnata is roughly ban-YAH-tah, not bag-na-ta.

It is most naturally understood as a general truth or general fact:

• Quando piove, l’erba è bagnata. = When it rains, the grass is wet.

Italian often uses the present tense for this kind of general statement.

If you wanted a more specific one-time event, context would usually make that clear, for example:

• Guarda fuori: quando piove, l’erba è subito bagnata.

But on its own, this sentence sounds general.