Learn Hardcore Vietnamese: Con mèo lại ngủ trên ghế. - The cat is sleeping on the chair again.

In this context, lại functions as an adverb meaning “again.” Placing it immediately before the verb is the normal Vietnamese word order for adverbs of aspect or frequency. So lại ngủ literally means “sleep again.”

Why do we say con mèo instead of just mèo, and why isn’t there a numeral like một?

Vietnamese often uses a classifier plus noun (here con + mèo) to talk about animals in a general or specific sense.

con mèo alone can mean “the cat” (definite) or “a cat” (indefinite) depending on context.
If you want to emphasize “one cat,” you’d say một con mèo.

Why is there no classifier before ghế (“chair”)? Can we add one?

In casual speech, classifiers for well-known or contextually clear items are often dropped. However, you can add the general classifier cái to be more explicit:

ngủ trên ghế (sleep on the chair)
ngủ trên cái ghế (sleep on the chair [more specific])

Why does trên ghế come after the verb ngủ? Could it come before?

Vietnamese typically places location phrases after the verb: V + (ở/trên) + location. So ngủ trên ghế is standard. If you move it before the verb, you’d need a prepositional marker:

Trên ghế, con mèo ngủ. (On the chair, the cat sleeps.)
That’s grammatical but more formal or literary.

What’s the difference between trên and ở when talking about location?

trên means “on” or “upon,” emphasizing surface contact.
ở is more general, meaning “at” or “in.”
You can say:
ngủ trên ghế = sleep on the chair (on its surface)
ngủ ở ghế (uncommon) would sound like “sleep at the chair” rather than on it.

Could we move lại to the end: Con mèo ngủ trên ghế lại?

No. In Vietnamese, aspect/frequency adverbs like lại, đã, vẫn must appear directly before the main verb, not at the end of the sentence.

What’s the difference between lại (again) and vẫn (still)?

lại indicates repetition: something happens again.
vẫn indicates continuation: something is still happening.
Example:
Con mèo lại ngủ trên ghế. → The cat is sleeping on the chair again.
Con mèo vẫn ngủ trên ghế. → The cat is still sleeping on the chair.

How do we express “the cat is sleeping again on the chair” in Vietnamese without using lại?

You could rephrase with a different structure or word:

Con mèo đang ngủ lại trên ghế. (adds đang for continuous aspect, but still uses lại)
Or explicitly state the repetition:
Con mèo ngủ trên ghế lần nữa. (The cat sleeps on the chair once more.)