Cổ động viên rất vui.

In Vietnamese, you usually do not use a copula like is/are before an adjective.

So:

• Cổ động viên rất vui = literally fan/supporter very happy

This is completely normal Vietnamese.

A very common beginner mistake is trying to add là here. But là is generally used when one noun is being identified as another noun:

• Anh ấy là cổ động viên. = He is a fan/supporter.

With adjectives such as vui, you normally do not use là:

• Anh ấy vui. = He is happy.
• Cổ động viên rất vui. = The fan / fans are very happy.

Rất means very and it goes before the adjective.

So the pattern here is:

• subject + rất + adjective

Examples:

• Cổ động viên rất vui. = very happy
• Cô ấy rất đẹp. = very beautiful
• Trời rất lạnh. = very cold

If you remove rất, the sentence still works:

• Cổ động viên vui. = The fan(s) are happy.

You can also replace rất with other degree words:

• hơi vui = somewhat happy
• khá vui = quite happy
• cực kỳ vui = extremely happy

In casual speech, people may also say rất là vui, which adds emphasis and sounds a bit more conversational.

In most learner-friendly explanations, vui is treated as an adjective meaning happy, joyful, or sometimes fun.

However, in Vietnamese grammar, adjectives can behave a bit like stative verbs when they are the main predicate of the sentence. That is why you can say:

• Cổ động viên rất vui.

without any extra word meaning to be.

So the short answer is:

• In vocabulary terms, vui is an adjective.
• In sentence structure, it can function directly as the predicate.

It can mean either happy or fun, depending on what it describes.

With a person, vui usually means:

• happy
• glad
• cheerful

With an event, place, or activity, it often means:

• fun
• enjoyable
• lively

So in Cổ động viên rất vui, because the subject is a person or group of people, vui most naturally means happy.

Compare:

• Cổ động viên rất vui. = The fan(s) are very happy.
• Buổi tiệc rất vui. = The party is very fun / enjoyable.

Usually, cổ động viên most strongly suggests a sports supporter or someone cheering for a team.

So it is especially natural in sports contexts, such as:

• football fans
• stadium supporters
• people cheering during a match

For a more general fan of a singer, actor, or celebrity, Vietnamese often uses:

• người hâm mộ

So:

• cổ động viên = supporter, especially in sports
• người hâm mộ = fan, admirer, follower more generally

In many contexts, cổ động viên feels more like supporter than just fan.

No. In cổ động viên, viên is part of the word itself, not a separate classifier.

It is better to learn cổ động viên as one fixed noun meaning fan/supporter.

You can think of it roughly as a compound:

• cổ động = to encourage, cheer on
• viên = person/member (in many Sino-Vietnamese compounds)

But in normal use, Vietnamese speakers treat cổ động viên as one unit.

So do not try to interpret it like:

• cổ động
  • some independent classifier viên

That is not how it works in this sentence.

By itself, it can be singular or plural. Vietnamese nouns usually do not change form for singular vs. plural.

So Cổ động viên rất vui could mean:

• The fan is very happy
• The fans are very happy
• Fans are very happy

Context tells you which one is intended.

If you want to be more specific, you can add words such as:

• một cổ động viên = one fan
• những cổ động viên = fans
• các cổ động viên = the fans / fans

Examples:

• Một cổ động viên rất vui. = One fan is very happy.
• Các cổ động viên rất vui. = The fans are very happy.

Vietnamese does not use articles the way English does.

So a bare noun like cổ động viên can mean:

• a fan
• the fan
• fans
• the fans

depending on context.

If Vietnamese wants to be more specific, it uses other tools, such as:

• numbers: một cổ động viên = one/a fan
• plural markers: các, những
• demonstratives: này, đó, kia

Examples:

• Cổ động viên này rất vui. = This fan is very happy.
• Các cổ động viên đó rất vui. = Those fans are very happy.

So the lack of a/the is completely normal.

A rough pronunciation guide is:

• cổ: like co with a dipping/rising tone
• động: like dong with a low, heavy, clipped tone
• viên: roughly vee-uhn with a level tone
• rất: a short syllable, roughly ruht or zut depending on accent, with a rising tone
• vui: roughly voo-ee with a level tone

A few useful notes:

• đ is pronounced like English d
• Vietnamese tones matter a lot, so the pitch shape is part of the word
• English-style approximations are only rough guides

Tone-wise:

• cổ = hỏi tone
• động = nặng tone
• viên = level tone
• rất = sắc tone
• vui = level tone

If you are learning pronunciation, it is best to listen to native audio and repeat the whole rhythm of the sentence, not just the individual consonants and vowels.