Trong phòng có một cái bàn sạch.

In this sentence có is an existential verb, similar to there is/there are or there exists.

• Trong phòng có một cái bàn sạch.
  → literally: In the room exists one clean table.

So here có does not mean to have/possess (as in Tôi có một cái bàn = I have a table), but rather introduces the existence of something in a place, just like English there is.

Vietnamese commonly puts the location at the beginning of an existential sentence to set the topic (what we are talking about) first:

• Trong phòng có một cái bàn sạch.
  → Topic: Trong phòng (In the room)
  → Comment: có một cái bàn sạch (there is a clean table)

You can also say:

• Có một cái bàn sạch trong phòng.

Both are correct.

• Trong phòng có ... emphasizes the room.
• Có một cái bàn sạch trong phòng emphasizes the existence of a clean table, then adds the location.

Một literally means one, but very often it works like the English a / an (indefinite article):

• một cái bàn sạch → a clean table / one clean table

Can it be omitted?

• If you say Trong phòng có cái bàn sạch, it usually sounds like you are talking about a specific/known clean table (more like the clean table), not introducing a new one.
• If you want to introduce an indefinite new object (like English There is a clean table in the room), một is the most natural choice.

So in this context, keeping một makes the sentence sound natural and indefinite, like English a.

Cái is a classifier (also called a measure word). Vietnamese typically needs a classifier when you:

• count a noun
• use a numeral like một before a countable noun

For objects like tables, chairs, books, etc., cái is a very common and neutral classifier.

Structure here is:

• một (numeral)
• cái (classifier)
• bàn (noun)

So một cái bàn is the normal, correct way to say one table / a table.
Một bàn is not standard in careful speech; learners should keep the classifier.

In Vietnamese, adjectives usually come after the noun they modify:

• bàn sạch → table clean → a clean table
• người cao → person tall → a tall person
• nhà mới → house new → a new house

So the pattern is:

• classifier + noun + adjective
  
  • một cái bàn sạch
  • một ngôi nhà đẹp (a beautiful house)

English: adjective + noun
Vietnamese: noun + adjective.

Sạch generally means clean, not dirty, free from dirt, dust, or stains.

• For a table (bàn), bàn sạch usually implies it has been wiped, there is no dust or stains.
• For a room: phòng sạch = the room is clean.

If you want to sound slightly more colloquial/emphatic, you might also hear:

• bàn sạch sẽ
  → also clean, often with a slightly stronger or more vivid feel.

In this sentence, bàn sạch is completely natural and just means a clean table.

In Vietnamese existential sentences with có, there is often no explicit grammatical subject in the same way as in English:

• English: There is a clean table in the room. (there acts as a dummy subject)
• Vietnamese: Trong phòng có một cái bàn sạch.

You can think of the pattern as:

• [Location] + có + [thing that exists]

The location (Trong phòng) works like the topic. The idea that a clean table exists there is expressed by có một cái bàn sạch, without needing a dummy subject like there.

Là is mainly used to link two noun phrases (or a pronoun and a noun phrase), like:

• Anh ấy là giáo viên. → He is a teacher.
• Đây là cái bàn. → This is a table.

With adjectives, Vietnamese normally does not use là:

• Cái bàn sạch. → The table is clean. (no là)
• Phòng này rộng. → This room is spacious.

In your sentence, có is the main verb (existence), and sạch is just an adjective describing bàn:

• Trong phòng (location)
• có (exist/existential verb)
• một cái bàn (noun phrase)
• sạch (adjective modifying bàn)

So there is no place for là here.

They have different structures and typical uses:

1. Trong phòng có một cái bàn sạch.

   • Existential: In the room, there is a clean table.
   • Introduces a new object that exists in that location.

2. Cái bàn trong phòng sạch. (more natural: Cái bàn trong phòng thì sạch.)

   • Descriptive: The table in the room is clean.
   • Cái bàn trong phòng is a specific table (the one in the room), and the sentence is describing its state.

So:

• Use Trong phòng có ... when you want to say there is/are ... in the room.
• Use Cái bàn trong phòng (thì) sạch. when you already know the table and are saying it is clean.

Yes:

• Trong phòng có một cái bàn rất sạch.
  → There is a very clean table in the room.

Here rất means very, and it modifies sạch.

You will also hear:

• rất là sạch in spoken Vietnamese, e.g. cái bàn rất là sạch.

In careful, standard grammar, rất sạch is enough and slightly more neutral; rất là sạch is common and natural in everyday speech, especially to add emphasis.

Yes, Ở trong phòng có một cái bàn sạch is also correct.

• trong phòng = in the room
• ở trong phòng = (located) in the room

Ở is a general preposition/verb for to be located at/in. Often:

• At the beginning of the sentence, trong phòng and ở trong phòng are both fine.
• Ở trong can sound a bit more explicit about location, but the meaning is the same here.

In many contexts, native speakers simply drop ở and just say trong phòng.

You can keep almost the same structure; Vietnamese often does not change the noun form for plural:

• Trong phòng có những cái bàn sạch.
  → explicitly plural (with những, meaning some / indicating plural)

or simply:

• Trong phòng có mấy cái bàn sạch. (if you mean a few clean tables)

Without những / mấy / vài / nhiều, the bare sentence:

• Trong phòng có cái bàn sạch.

usually sounds singular and specific (more like the clean table), so to clearly express plural clean tables, use a plural marker like những, mấy, vài, nhiều, depending on what you want to say.

Word by word with tone names (Northern description):

• Trong – no tone mark → mid-level tone
• phòng – phòng with huyền (falling) → phòng
• có – sắc (rising) → có
• một – nặng (heavy, glottal stop at the end) → một
• cái – sắc (rising) → cái
• bàn – huyền (falling) → bàn
• sạch – nặng (heavy, short/abrupt) → sạch

Practising slowly with clear tone and final consonants (ng, t, n, ch) will help the whole sentence sound natural.