Όχι, θέλω τσάι.

A natural pronunciation is roughly:

• Όχι = OH-khee
• θέλω = THE-lo
• τσάι = TSA-ee

So the whole sentence sounds like:

OH-khee, THE-lo TSA-ee.

A few notes:

• χ in όχι is not an English h. It is a harsher sound, like the ch in German ich or a softer version of the ch in Scottish loch.
• θ in θέλω sounds like th in think, not like th in this.
• τσ is like ts in cats.

Όχι means no.

In this sentence, it is rejecting or correcting something implied before it. For example:

• Coffee?
• Όχι, θέλω τσάι.
• No, I want tea.

So όχι is not part of the verb phrase. It is a separate response word.

The comma shows a pause after no.

Greek commonly writes this kind of sentence with a comma because Όχι is functioning as a separate response before the main statement:

• Όχι, θέλω τσάι. = No, I want tea.

This is similar to English punctuation in No, I want tea.

Because Greek usually does not need the subject pronoun when the verb already shows who the subject is.

In θέλω, the ending tells you it means I want.

So:

• θέλω = I want

You could say εγώ θέλω for emphasis, but normally it is unnecessary.

Compare:

• θέλω τσάι = I want tea
• εγώ θέλω τσάι = I want tea / I’m the one who wants tea

The second one sounds more emphatic.

θέλω means I want.

It is the 1st person singular form of the verb θέλω = to want.

Some common forms are:

• θέλω = I want
• θέλεις = you want
• θέλει = he/she/it wants
• θέλουμε or θέλομε = we want
• θέλετε = you (plural / polite) want
• θέλουν or θέλουνε = they want

So in θέλω τσάι, the verb alone already tells us the subject is I.

Greek does have an indefinite article, but it is often omitted with food, drink, and general objects when the meaning is obvious.

So:

• θέλω τσάι = I want tea
• θέλω ένα τσάι = I want a tea / I want one tea

The version without ένα sounds very natural if you are simply saying what you want to drink.
The version with ένα is also possible, especially if you are ordering one tea.

Both are possible, but they mean slightly different things in use.

• θέλω τσάι = I want tea
• θέλω το τσάι = I want the tea

So:

• no article = tea in a general sense
• το = a specific tea, something already known

For example:

• Τι θέλεις; — Θέλω τσάι.
  What do you want? — I want tea.

But:

• Θέλεις καφέ ή το τσάι σου; — Θέλω το τσάι μου.
  Do you want coffee or your tea? — I want my tea.

τσάι is a neuter noun.

Its basic form is:

• το τσάι = the tea

That is why, if you use the definite article, it is το, not ο or η.

As a learner, it is useful to memorize nouns together with their article:

• το τσάι

Yes, Greek word order is more flexible than English, but not every order sounds equally natural in every situation.

The neutral sentence is:

• Όχι, θέλω τσάι.

You may also hear:

• Όχι, τσάι θέλω.

This puts extra emphasis on tea, as if contrasting it with something else:

• No, tea is what I want.

So the original version is the most straightforward and natural for a beginner.

Greek uses accent marks to show the stressed syllable.

• Όχι: stress on the first syllable
• θέλω: stress on the first syllable
• τσάι: stress on the first part

These marks are important in normal Greek spelling.

In Όχι, the capital letter still keeps its accent because it begins the sentence.
In τσάι, the accent helps show the stress, and the spelling also signals that the vowel sounds are pronounced separately.

Θέλω τσάι is completely correct and common, but depending on the situation, Greek may use softer or more polite wording.

For example:

• Θέλω τσάι. = I want tea.
• Θα ήθελα τσάι. = I would like tea.

If you are ordering in a café or speaking politely, Θα ήθελα τσάι may sound more polite, like English I’d like tea.

But θέλω τσάι is still very common in everyday speech, especially in casual situations.

It is neutral and everyday.

• Όχι is a normal way to say no
• θέλω is normal everyday speech
• τσάι is the ordinary word for tea

So the sentence works well in casual conversation.
If you want to sound more polite, especially in service situations, you might choose:

• Όχι, θα ήθελα τσάι.

That is a softer version of the same basic idea.