Learn Hardcore Portuguese (Portugal): O Pedro senta-se ao lado da Ana no cinema.

This sounds natural and neutral in European Portuguese, especially in spoken language. In more formal or literary writing, the article is sometimes dropped, but in everyday speech O Pedro / a Ana is the norm.

The article also agrees with the gender of the name:

masculine names → o (singular), os (plural)
feminine names → a (singular), as (plural)

What exactly does senta-se mean, and what is the role of -se?

The infinitive form is sentar-se, a pronominal (reflexive) verb.

sentar (without se) = to seat (someone)
- e.g. O professor senta os alunos. = The teacher seats the students.
sentar-se (with se) = to sit down (the subject sits themself)

In O Pedro senta-se ao lado da Ana, senta-se means Pedro sits down or Pedro is sitting (himself) down.

The se is a clitic pronoun that refers back to the subject (Pedro), but in English we usually don’t translate it as himself; we just say sits (down).

Can I say O Pedro senta ao lado da Ana without se?

In standard European Portuguese, no. That would sound strange if there is no object.

sentar without se normally needs a direct object:
- O Pedro senta a Ana. = Pedro seats Ana (he helps her sit down).

When there is no direct object and the person themself is sitting, you practically always use sentar-se:

O Pedro senta-se ao lado da Ana. ✅ (natural)
O Pedro senta ao lado da Ana. ❌ (sounds wrong in EP in this meaning)

So in your sentence, se is necessary.

Why is the verb in the simple present (senta-se) and not a continuous form like está a sentar-se?

Portuguese uses the simple present much more than English for actions happening “now”.

O Pedro senta-se ao lado da Ana.
→ can mean Pedro sits down next to Ana (right now) or Pedro sits next to Ana (whenever they go to the cinema), depending on context.

The continuous form está a sentar-se (is sitting down) is possible but often feels more focused on the ongoing physical movement of sitting down at that exact moment.

Very often, to describe the state of already being seated, Portuguese prefers:

O Pedro está sentado ao lado da Ana. = Pedro is seated / sitting next to Ana.

Why is it senta-se and not se senta?

This is about clitic pronoun placement, which differs between European and Brazilian Portuguese.

In European Portuguese main positive sentences that simply start with a subject and verb (like yours), the default is enclisis: the clitic goes after the verb:

O Pedro senta-se ao lado da Ana.

You get proclisis (se senta) mainly when something attracts the pronoun to the left, for example:

Negation: O Pedro não se senta ao lado da Ana.
Certain adverbs/conjunctions: Quando chega, o Pedro se senta ao lado da Ana.

In Brazilian Portuguese, se senta (proclisis) is much more common even in simple sentences. But in standard European Portuguese, senta-se is the normal form here.

What does ao lado literally mean, and how is ao formed?

Ao lado de is a common expression meaning next to / beside.

lado = side
de = of
a + o → ao (preposition + article contraction)

So literally:

ao lado da Ana = at the side of Ana → next to Ana

The structure is:

ao = a (to/at) + o (the, masculine singular)
lado = side
de
- a (Ana) = da Ana

Why is it da Ana and not de Ana or de a Ana?

Two things happen here:

Contraction of preposition + article
- de + a → da
  So de a Ana becomes da Ana.
Article with names
In European Portuguese, people’s names usually take the article, so you normally get da Ana, not de Ana, in everyday speech.

So the normal, natural form is:

ao lado da Ana = next to Ana

De Ana without the article is possible in special contexts (very formal style, some set expressions, lists, etc.), but da Ana is what you want here.

What exactly does no cinema mean, and how is no formed?

No is another contraction:

em (in/at) + o (the, masculine singular) → no

So:

no cinema = in the cinema / at the cinema
(and often corresponds to English at the movies.)

Structure:

em = in/at
o cinema = the cinema
em + o → no

So the whole phrase is:
O Pedro senta-se ao lado da Ana no cinema.
= Pedro sits next to Ana at the cinema.

Why is cinema singular in Portuguese when in English we often say “at the movies”?

Portuguese doesn’t use a plural here; it uses the singular noun cinema:

ir ao cinema = to go to the cinema / to the movies
estar no cinema = to be at the cinema / at the movies

The plural idea (movies) is already implied by the context: the place where films are shown. Grammatically, though, it stays singular: o cinema / no cinema.

Can I change the word order, for example: No cinema, o Pedro senta-se ao lado da Ana?

Yes. Word order is quite flexible for emphasis or style.

O Pedro senta-se ao lado da Ana no cinema.
No cinema, o Pedro senta-se ao lado da Ana.

Both are correct. The second version slightly emphasizes no cinema (as opposed to some other place), but the basic meaning is the same.

You just need to keep the internal groups together:

O Pedro (subject)
senta-se (verb)
ao lado da Ana (complement)
no cinema (location)

How would I say “Pedro sat next to Ana at the cinema” (past tense)?

Use the pretérito perfeito (simple past) of sentar-se:

O Pedro sentou-se ao lado da Ana no cinema.
= Pedro sat next to Ana at the cinema.

Only the verb changes:

present: senta-se
past: sentou-se

Is there a difference between ao lado de, perto de, and junto de?

Yes, slight differences:

ao lado de = right next to, beside
- very close, typically immediately next to someone/something.
perto de = near, close to
- close, but not necessarily immediately next to.
junto de = close to / by / next to
- often similar to perto de, sometimes slightly more formal or bookish, or used in fixed expressions.

In your sentence, ao lado da Ana is the most natural if you mean he is actually sitting right next to her, in the adjacent seat.