Bãi cỏ ở trong công viên.

Not exactly. cỏ means grass, but bãi cỏ means a grassy area, lawn, or patch/expanse of grass.

So in this sentence, bãi cỏ is a noun phrase, not just the material grass in general.

• cỏ = grass
• bãi cỏ = lawn / grassy area

The word bãi often gives the idea of an area or stretch of something.

Here, ở works as a location verb meaning to be located at/in.

Vietnamese does not use to be in exactly the same way English does. In a sentence like this, ở is what links the subject to its location:

• Bãi cỏ = the lawn / grassy area
• ở trong công viên = is in the park

So ở is very important here because it tells you that the sentence is about location.

They do overlap in meaning, but they have different jobs.

• ở = indicates location, roughly to be at/in
• trong = inside / in

So ở trong công viên is literally something like is located inside the park.

In natural English, we usually just say is in the park, but Vietnamese often uses both parts together.

Usually, that would not sound like a complete sentence in the same way.

• Bãi cỏ ở trong công viên = The lawn is in the park.
• Bãi cỏ trong công viên = more like the lawn in the park

Without ở, it often sounds like a noun phrase that describes which lawn you mean, rather than making a full statement about where the lawn is.

Yes, often you can.

• ở công viên = at/in the park
• ở trong công viên = in/inside the park

The version with trong makes the idea of being inside the park area a bit clearer.
The version without trong can sound slightly more general.

So both can be natural, but ở trong công viên is a little more explicit.

The word order is:

subject + location verb + place

So:

• Bãi cỏ = subject
• ở trong công viên = location part

This is actually quite close to English:

• The lawn
  • is in the park

So the sentence structure is not especially difficult for English speakers once you know what ở does.

Vietnamese usually does not use articles the way English does.

So bãi cỏ could mean:

• a lawn
• the lawn
• the grassy area

The exact meaning depends on context.

If you want to make a more explicit, you can say:

• một bãi cỏ = a lawn / one lawn

But in many normal sentences, Vietnamese simply leaves articles unstated.

Not in the usual sense of a separate classifier like cái, con, or chiếc.

Here, bãi cỏ is best learned as a unit meaning lawn / grassy area.
So bãi is part of the noun expression itself.

For example:

• một bãi cỏ = a lawn / a grassy area

You would normally not say một cái bãi cỏ.

The tones are:

• bãi — ngã tone
• cỏ — hỏi tone
• ở — hỏi tone
• trong — level tone
• công — level tone
• viên — level tone

A learner may especially need to pay attention to:

• bãi
• cỏ
• ở

Those are the words with marked tones that can strongly affect meaning if pronounced incorrectly.

You could, but a more natural Vietnamese sentence would be:

Trong công viên có một bãi cỏ.

That pattern uses có to express existence: there is / there are.

So the difference is:

• Bãi cỏ ở trong công viên. = The lawn is in the park.
• Trong công viên có một bãi cỏ. = There is a lawn in the park.

The first sentence talks about the location of the lawn.
The second introduces the lawn as something that exists in the park.