Després de caminar, menjo un plàtan i bec suc de taronja.

Because després de is the normal Catalan pattern for after when it is followed by a verb.

• després de caminar = after walking
• després de dinar = after eating lunch
• després de treballar = after working

So de is required here before the infinitive caminar.

Caminar is the infinitive, the basic dictionary form of the verb, meaning to walk.

After després de, Catalan often uses an infinitive where English might use:

• after walking
• after I walk
• after having walked

So després de caminar is a very natural Catalan structure.

Catalan often omits subject pronouns because the verb ending already shows who the subject is.

• menjo = I eat
• bec = I drink

So adding jo is usually unnecessary unless you want emphasis or contrast.

For example:

• Jo menjo un plàtan, però ell menja una poma.
  = I eat a banana, but he eats an apple.

In your sentence, leaving out jo sounds completely normal.

They are both first person singular present tense forms.

• menjo comes from menjar = to eat
• bec comes from beure = to drink

So:

• menjo = I eat / I am eating
• bec = I drink / I am drinking

Catalan present tense can often cover both the simple present and the present progressive, depending on context.

Because beure is an irregular verb.

Its present tense forms are:

• bec = I drink
• beus = you drink
• beu = he/she drinks
• bevem = we drink
• beveu = you all drink
• beuen = they drink

So bec is just the correct irregular I form.

This is a countable vs. uncountable difference.

• un plàtan: a banana is a countable item
• suc de taronja: orange juice is usually treated as an uncountable substance

So:

• menjo un plàtan = I eat a banana
• bec suc de taronja = I drink orange juice

You can say un suc de taronja in some contexts, especially if you mean:

• a glass of orange juice
• an orange juice as a drink order

But in a neutral sentence, bec suc de taronja is very natural.

Catalan commonly uses de + noun to describe the type or flavor of something.

So:

• suc de taronja = orange juice
  literally, juice of orange
• gelat de xocolata = chocolate ice cream
• pastís de poma = apple pie

This is a very common structure in Catalan.

I means and.

So:

• menjo un plàtan i bec suc de taronja
  = I eat a banana and drink orange juice

It simply joins the two actions.

Yes. Menjo and bec are both in the present tense.

Depending on context, the Catalan present can mean:

• a current action: I’m eating / I’m drinking
• a routine or habit: I eat / I drink

So the sentence could describe:

• what the speaker is doing now, or
• something they usually do after walking

Both are possible unless context makes one clearer.

Yes, Catalan word order is fairly flexible, although the original version is very natural.

Your sentence:

• Després de caminar, menjo un plàtan i bec suc de taronja.

You could also say:

• Menjo un plàtan i bec suc de taronja després de caminar.

Both are correct. Putting després de caminar first gives the time/context first, which is very common.

The accent marks show stress and sometimes vowel quality.

• després is stressed on the last syllable
• plàtan is stressed on the first syllable

In Catalan, written accents are important because they help you pronounce the word correctly and can sometimes distinguish words.

So you should learn them as part of the spelling, not as optional marks.

A rough pronunciation guide for a learner is:

des-PRÉS de ca-mi-NAR, MEN-ju un PLA-tan i BEK suk de ta-RON-ja

A few notes:

• j in menjo sounds like the s in measure
• c in bec sounds like k
• ny does not appear here, but taronja ends with that soft j sound again
• x does not appear here either, so this sentence is fairly beginner-friendly

Pronunciation varies somewhat by dialect, but this approximation is a useful start.

Yes, you can, but it adds a slightly different nuance.

• menjo un plàtan = I eat a banana
• em menjo un plàtan = I eat up a banana / I have a banana

Using em with some food verbs can make the action feel a bit more personal, complete, or colloquial. But the plain menjo un plàtan is perfectly normal and probably the most neutral choice for a learner.

They are similar, but not exactly identical in nuance.

• després de caminar = after walking
• després d’haver caminat = after having walked

The second version sounds a bit more explicit or formal because it highlights that the action has been completed. In everyday Catalan, després de caminar is usually the simpler and more natural option.