Khỉ ăn ở trên cây.

The sentence is built like this:

• Khỉ = subject
• ăn = verb
• ở trên cây = location phrase

So the pattern is basically Subject + Verb + Place.

That is very common in Vietnamese. English often does something similar too, as in Monkeys eat in the tree or The monkey is eating in the tree.

Vietnamese does not use articles the way English does. A bare noun like khỉ can mean:

• a monkey
• the monkey
• monkeys
• or monkeys in general

Context tells you which one is meant.

If you want to be more specific, Vietnamese can add other words, for example:

• một con khỉ = a/one monkey
• con khỉ = the monkey or a particular monkey
• những con khỉ / các con khỉ = monkeys

By itself, khỉ does not clearly mark singular or plural. It can be understood as:

• one monkey
• several monkeys
• monkeys as a kind of animal

Vietnamese often leaves number unmarked unless it matters.

If the speaker wants to make it clear, they might say:

• một con khỉ = one monkey
• hai con khỉ = two monkeys
• những con khỉ = monkeys

Because classifiers are not required in every sentence.

With animals, con is the usual classifier, but it is most often used when the noun is being:

• counted
• specified
• pointed out
• modified by words like this, that, one, two, etc.

So:

• một con khỉ = one monkey
• con khỉ này = this monkey

But in a simple general statement, Vietnamese often uses just the bare noun:

• Khỉ ăn ở trên cây

That sounds natural.

Vietnamese verbs do not change form for tense the way English verbs do.

So ăn stays ăn whether the action is present, past, future, habitual, or generic. Time is usually understood from:

• context
• time words
• aspect markers

For example:

• Khỉ đã ăn ở trên cây = the monkey/monkeys ate / have eaten in the tree
• Khỉ đang ăn ở trên cây = the monkey/monkeys are eating in the tree
• Khỉ sẽ ăn ở trên cây = the monkey/monkeys will eat in the tree

Without extra markers, the sentence is often understood as a simple present, habitual action, or a general statement.

Yes, you often can.

Here, ở helps mark location.
trên gives the spatial relationship: on / above / up on.
So ở trên cây means something like at/on/in the tree, depending on the natural English translation.

Many Vietnamese sentences can omit ở after an action verb if the place phrase is already clear:

• Khỉ ăn ở trên cây
• Khỉ ăn trên cây

Both can be natural. The version with ở can sound a little more explicitly locational.

This is a very common learner question.

Vietnamese often uses trên because the animal is understood as being on the branches / up on the tree, not literally inside it.

So even when English naturally says in the tree, Vietnamese may say:

• trên cây = literally on the tree / up on the tree

This is one of those places where the two languages picture space a little differently.

Vietnamese can leave out the object when it is:

• unknown
• obvious
• not important
• meant in a general sense

So ăn does not always need an expressed object.

In this sentence, the focus is on the action and location, not on the food. The idea is simply that the monkey or monkeys are eating there.

If you want to add the thing being eaten, you can:

• Khỉ ăn trái cây ở trên cây = The monkeys eat fruit in the tree.

Because cây comes inside the phrase ở trên cây, which is a place expression.

So the structure is:

• ăn = eat
• ở trên cây = on/in the tree

That means cây is the location, not the direct object.

If cây were the thing being eaten, you would expect it directly after ăn, like:

• Khỉ ăn cây

That would mean The monkey eats the tree/plants, which is a very different structure.

A rough breakdown is:

• khỉ — has the hỏi tone
• ăn — level tone
• ở — also hỏi tone
• trên — level tone
• cây — huyền tone

A few pronunciation notes:

• kh is not a simple English k. It is a breathy sound, somewhat like the ch in Scottish loch or German Bach.
• ă in ăn is short.
• tr in trên may sound different depending on region.
• Tones matter a lot, especially on khỉ, ở, and cây.

If you are learning Northern Vietnamese, those tone distinctions are especially important. In Southern Vietnamese, some tone values are pronounced differently.

You could add words for number, definiteness, or aspect. For example:

• Một con khỉ ăn ở trên cây. = One monkey is eating in the tree.
• Con khỉ ăn ở trên cây. = The monkey is eating in the tree.
• Những con khỉ ăn ở trên cây. = The monkeys are eating in the tree.
• Con khỉ đang ăn ở trên cây. = The monkey is eating in the tree right now.

So the original sentence is natural, but it is also fairly bare and context-dependent, which is very normal in Vietnamese.