Ele é capaz de ajudar.

Because capaz is describing a more general or inherent quality, not a temporary condition.

• ser → used for identity, characteristics, and more permanent traits
• estar → used for temporary states or conditions

So:

• Ele é capaz de ajudar. = He is capable of helping.

Using está capaz would sound unnatural in this context in standard Brazilian Portuguese.

Capaz de means capable of or able to.

In this sentence:

• ele = he
• é capaz de = is capable of / is able to
• ajudar = to help

A useful pattern to remember is:

• ser capaz de + infinitive

Examples:

• Ela é capaz de resolver isso. = She is capable of solving that.
• Você é capaz de fazer isso? = Are you able to do that?

Because capaz normally connects to a following verb with the preposition de.

So the structure is:

• ser capaz de + infinitive

Examples:

• Ele é capaz de ganhar.
• Somos capazes de entender.

You generally should not say Ele é capaz ajudar. The de is part of the grammar pattern.

Because after ser capaz de, Portuguese uses the infinitive form of the next verb.

So:

• é capaz de ajudar
• é capaz de estudar
• é capaz de vencer

This is similar to English able to help, where help stays in its base form.

Yes. It agrees in number, but not in masculine/feminine singular the way many adjectives do.

Forms:

• capaz = singular
• capazes = plural

Examples:

• Ele é capaz de ajudar.
• Ela é capaz de ajudar.
• Eles são capazes de ajudar.
• Elas são capazes de ajudar.

Notice:

• singular masculine: capaz
• singular feminine: capaz
• plural masculine/feminine: capazes

Yes, you can, but the meaning is slightly different.

• Ele pode ajudar. = He can help.
  This often focuses on possibility, permission, or immediate ability.
• Ele é capaz de ajudar. = He is capable of helping.
  This emphasizes capacity, competence, or suitability.

So é capaz de can sound a bit more about someone's ability or potential, while pode is often more direct and common in everyday speech.

In many cases, Portuguese can drop the subject pronoun, but here ele is often kept for clarity.

You could say:

• É capaz de ajudar.

But without context, that could sound incomplete or ambiguous: Who is capable of helping?

Because é could refer to he, she, or you depending on context, speakers often include the pronoun when needed:

• Ele é capaz de ajudar.
• Ela é capaz de ajudar.
• Você é capaz de ajudar.

A rough pronunciation guide is:

EH-lee eh kah-PAHZ djee ah-zhoo-DAR

A few notes:

• Ele → EH-lee
• é → like eh
• capaz → stress on the last syllable: ka-PAHZ
• de often sounds like djee before a vowel in many Brazilian accents
• ajudar → ah-zhoo-DAR

The strongest stresses are usually on:

• capaz
• ajudar

Yes. In this sentence, capaz clearly means capable.

But in some parts of Brazil, especially in southern speech, capaz! can also be an expression meaning something like:

• No way!
• Really?
• Come on!

That is a separate, idiomatic use. It does not change the meaning of this sentence.

Here, Ele é capaz de ajudar is just the normal adjective meaning he is capable of helping.

Put não before the verb é:

• Ele não é capaz de ajudar.

This means:

• He is not capable of helping.

Pattern:

• subject + não + ser + capaz de + infinitive

Examples:

• Ela não é capaz de entender isso.
• Nós não somos capazes de fazer tudo sozinhos.

In Brazilian Portuguese, you usually keep the same word order and use intonation when speaking:

• Ele é capaz de ajudar?

In writing, the question mark shows it is a question.

Depending on context, this can mean:

• Is he capable of helping?
• Is he able to help?

You could also ask more naturally in conversation:

• Será que ele é capaz de ajudar?

This can sound a little more tentative, like:

• Do you think he is capable of helping?

Yes, it is grammatical and natural. But depending on the situation, Brazilians might also choose other common expressions, such as:

• Ele pode ajudar. = He can help.
• Ele consegue ajudar. = He manages to help / is able to help.
• Ele tem condições de ajudar. = He is in a position to help.

Ele é capaz de ajudar sounds perfectly normal when you want to highlight ability or capability a bit more explicitly.