El barco se mueve despacio cuando las olas son fuertes.

Because mover and moverse are used differently.

• mover = to move something
  • Muevo la silla = I move the chair
• moverse = to move / to move oneself / to shift
  • El barco se mueve = The boat moves / shifts / rocks

So in this sentence, the boat is not moving another object; the boat itself is moving. That is why Spanish uses the pronominal form moverse.

Despacio means slowly.

It is an adverb, so it describes how the boat moves:

• se mueve despacio = moves slowly

A useful point: despacio does not change form. It stays the same no matter what noun or verb is in the sentence.

You could also hear lentamente, which also means slowly, but despacio is very common and natural in everyday Spanish.

Spanish uses the definite article very often, including with nouns that are being talked about in a general or specific way already understood from context.

Here, El barco simply means the boat.

In many cases, Spanish uses articles more often than English does. For example, with general statements, body parts, and many common nouns, Spanish tends to include el/la/los/las where English might not.

In this sentence, El barco sounds completely normal and expected.

Because olas is a plural feminine noun.

• singular: la ola = the wave
• plural: las olas = the waves

The article has to match the noun in gender and number:

• la
  • singular feminine
• las
  • plural feminine

So las olas is the correct form.

Because the adjective must agree with the noun it describes.

Here, fuertes describes olas:

• ola = singular
• olas = plural

So:

• la ola es fuerte = the wave is strong
• las olas son fuertes = the waves are strong

Notice that fuerte becomes fuertes in the plural. It does not change for masculine/feminine in the singular, but it does change for number.

Because ser is normally used to describe a characteristic or quality like strong in this context.

• las olas son fuertes = the waves are strong

With weather and sea conditions, Spanish commonly uses ser for this kind of description.

Very broadly:

• ser = what something is like
• estar = how something is / its state or condition

For waves, son fuertes is the natural choice here.

The present tense is used here to express a general fact or a typical situation.

So the sentence means something like:

• The boat moves slowly when the waves are strong
• The boat moves slowly whenever the waves are strong

This is a common use of the present tense in both Spanish and English for general truths, repeated situations, or typical behavior.

Here cuando can be understood as when or whenever, depending on context.

In this sentence, it sounds like a general condition:

• El barco se mueve despacio cuando las olas son fuertes.
• The boat moves slowly when / whenever the waves are strong.

Because the sentence describes a regular or general situation, Spanish uses cuando with the indicative here: son.

Yes. Cuando can be followed by either the indicative or the subjunctive, depending on meaning.

• Indicative: for habitual, real, or repeated situations
  
  • cuando las olas son fuertes
• Subjunctive: for future or not-yet-real situations
  
  • cuando las olas sean fuertes

Compare:

• El barco se mueve despacio cuando las olas son fuertes.
  = This is what happens in general.
• El barco se moverá despacio cuando las olas sean fuertes.
  = It will move slowly when the waves are strong.

So in your sentence, the indicative is used because it describes a general fact.

Yes, you could, but the meaning changes slightly.

• El barco se mueve despacio = The boat moves slowly / shifts slowly / rocks slowly
• El barco va despacio = The boat goes slowly / is travelling slowly

So:

• moverse focuses on the boat’s movement itself
• ir focuses more on its progress or speed

In the context of strong waves, se mueve despacio can suggest the boat’s motion through the water or its slow rocking/movement. Va despacio would sound more like slow travel.

Yes, but the most natural place here is after the verb:

• El barco se mueve despacio cuando las olas son fuertes.

Other orders are possible for emphasis, but they are less neutral:

• Despacio se mueve el barco cuando las olas son fuertes.
• El barco, cuando las olas son fuertes, se mueve despacio.

So for a learner, the original order is the best and most standard one to use.

Mueve is pronounced roughly like MWEH-beh.

A few helpful points:

• ue in mueve forms a glide, so it sounds like mwe
• v and b are pronounced very similarly in Spanish
• stress falls on mue-: MUE-ve

So:

• mueve ≈ MWEH-beh

Yes, it is natural and correct.

It sounds like a normal descriptive sentence in standard Spanish, including Spanish from Spain. The vocabulary and grammar are straightforward and common:

• el barco
• se mueve
• despacio
• cuando
• las olas
• son fuertes

A native speaker would understand it immediately.