Mi abuelo anda despacio porque le duele la rodilla.

Both andar and caminar can mean to walk.

In this sentence, anda despacio sounds very natural and means something like he walks / he moves slowly. In Spain, andar is very commonly used in everyday speech.

A rough difference is:

• andar = to walk, to go around, to move about
• caminar = to walk, often a little more specifically focused on the act of walking

So:

• Mi abuelo anda despacio = My grandfather walks/moves slowly
• Mi abuelo camina despacio would also be correct

Despacio means slowly.

It is an adverb, so it describes how someone does something.

Here it describes anda:

• anda = walks / moves
• despacio = slowly

So anda despacio = walks slowly.

In Spanish, adverbs like despacio often come after the verb.

So:

• anda despacio = walks slowly

That word order is very normal in Spanish. English can also do this: walks slowly.

These are different expressions:

• porque = because
• por qué = why

In your sentence, the second part gives the reason, so Spanish uses porque:

• Mi abuelo anda despacio porque le duele la rodilla.
• My grandfather walks slowly because his knee hurts.

If you were asking a question, you would use por qué:

• ¿Por qué anda despacio tu abuelo?
• Why does your grandfather walk slowly?

Spanish often expresses pain differently from English.

The structure is:

• le = to him
• duele = hurts
• la rodilla = the knee

Literally, it is closer to:

• The knee hurts to him

Natural English changes that to:

• His knee hurts
• or His knee is hurting

This is the same pattern as verbs like gustar:

• Le gusta el café = He likes coffee
• literally: Coffee is pleasing to him

So doler works in a similar way.

Le is an indirect object pronoun. Here it means to him.

In this sentence, it tells you who is affected by the pain:

• le duele la rodilla = his knee hurts / the knee hurts him

Because the sentence starts with Mi abuelo, we know le refers to my grandfather.

Other forms would be:

• me duele = my ... hurts
• te duele = your ... hurts
• le duele = his/her/your (formal) ... hurts
• nos duele = our ... hurts
• les duele = their/your plural ... hurts

With body parts, Spanish often uses:

• an indirect object pronoun to show whose
• plus the definite article (el, la, los, las)

So instead of saying his knee, Spanish often says:

• le duele la rodilla

That is the normal pattern.

Examples:

• Me duele la cabeza = My head hurts
• Le duelen los pies = His/Her feet hurt
• Nos lavamos las manos = We wash our hands

Spanish usually avoids su rodilla here because le already tells you whose knee it is.

Because the subject of doler is the thing that hurts.

Here, the subject is:

• la rodilla = the knee

That is singular, so the verb is singular:

• la rodilla duele
• therefore: le duele la rodilla

If the body part were plural, the verb would be plural too:

• le duelen las rodillas = his knees hurt
• me duelen los pies = my feet hurt

Yes, it could be plural, but the sentence as written says only one knee hurts:

• le duele la rodilla = his knee hurts

If both knees hurt, Spanish would usually say:

• le duelen las rodillas = his knees hurt

So the singular/plural depends on the meaning.

The present tense in Spanish can do both, depending on context.

So Mi abuelo anda despacio can mean:

• My grandfather walks slowly in general
• or My grandfather is walking slowly right now

In this sentence, because of porque le duele la rodilla, it may sound like a current situation, but it could also describe a general tendency. Context would decide.

Yes, grammatically that is possible, but it is less natural in many everyday situations.

Spanish often uses the simple present where English might use is walking:

• Mi abuelo anda despacio = My grandfather is walking slowly / walks slowly

The form está andando is possible, but Spanish usually does not need the progressive as often as English does.

So the original sentence sounds more natural in many contexts.

If the person is already clear from context, Spanish can sometimes leave out the subject:

• Anda despacio porque le duele la rodilla.

That means:

• He walks slowly because his knee hurts.

Spanish often drops subject pronouns like él, but here mi abuelo is a noun, not a pronoun. It is included because the speaker wants to identify exactly who they are talking about.

So:

• Mi abuelo anda despacio... = clearer, more explicit
• Anda despacio... = possible if the listener already knows who he is

Duele is pronounced roughly like DWEH-leh.

A few pronunciation notes:

• due- sounds like dwe-
• -le sounds like leh

So:

• duele ≈ DWEH-leh

And rodilla in most of Spain is usually pronounced roughly ro-DEE-ya. The ll is commonly pronounced like y, although pronunciation can vary by region.