Todos seguem o líder.

Here todos is being used as a pronoun, not as an adjective before a noun.

It means everyone, all of them, or all people, depending on context.

A few useful comparisons:

• todos seguem o líder = everyone follows the leader
• todos os alunos seguem o líder = all the students follow the leader

By itself, todos refers to a group of people in a general way.

Because todos is plural.

The verb has to agree with the subject:

• ele/ela segue = he/she follows
• eles/elas seguem = they follow

Since todos refers to multiple people, Portuguese uses the 3rd person plural form: seguem.

The infinitive is seguir, meaning to follow.

In the present tense:

• eu sigo = I follow
• você/ele/ela segue = you/he/she follows
• nós seguimos = we follow
• vocês/eles/elas seguem = you all/they follow

So in Todos seguem o líder, seguem is the they form.

Portuguese often leaves out subject pronouns when the verb already makes the subject clear.

So:

• Todos seguem o líder
• Eles seguem o líder

Both are grammatical, but eles is unnecessary unless you want emphasis or contrast.

Because seguem already shows a plural subject, Portuguese does not need they the way English usually does.

Yes.

Use todos for:

• a mixed group
• a group of men
• a group of unspecified people

Use todas for:

• a group made up only of women or girls

Examples:

• Todos seguem o líder.
• Todas seguem a líder.

So the form changes according to grammatical gender.

In normal Portuguese, singular countable nouns often need an article when they are specific.

So:

• o líder = the leader

In this sentence, the speaker is talking about a particular leader, so o is natural and expected.

Saying Todos seguem líder would sound unnatural in standard Portuguese.

The verb seguir normally takes a direct object, so it does not need a preposition here.

• seguir alguém = to follow someone
• seguir o líder = to follow the leader

So:

• Todos seguem o líder. = correct
• Todos seguem ao líder. = not the normal pattern for this meaning

This is different from some Portuguese verbs that do require a preposition.

The accent shows where the stress goes.

So líder is stressed on the first syllable:

• LÍ-der

Without the accent, a learner might guess the stress incorrectly.

The accent is part of the correct spelling.

The normal, neutral order is:

• subject + verb + object
• Todos seguem o líder.

That is the best pattern for a beginner to learn first.

Other versions are possible in certain contexts, but they are less neutral:

• Eles seguem o líder. = adds an explicit subject
• Todos o seguem. = uses o as a pronoun for him or the leader; this sounds more formal or written

So the original sentence is the most straightforward everyday structure.

A rough English-friendly approximation is:

TOH-doos SEH-gem oo LEE-der

A few notes:

• todos: the final -os often sounds like -us
• seguem: the -em ending is nasal in Brazilian Portuguese
• líder: stress is on LÍ-

So the most important stress points are:

• TO-dos
• SE-guem
• LÍ-der

If you want to sound more natural, pay special attention to the nasal ending in seguem.