Tôi ngồi bên Lan.

Tôi means I / me, but Vietnamese pronouns depend a lot on age, relationship, and social context.

• tôi is a neutral, polite, and fairly safe way to say I
• it is common in careful speech, formal situations, or textbook examples
• in real life, people often use other pronouns based on who they are talking to

So Tôi ngồi bên Lan is completely correct, especially as a neutral learning sentence.

Ngồi means to sit or to be sitting.

Vietnamese verbs do not change form the way English verbs do, so ngồi can cover several English ideas depending on context:

• sit
• am sitting
• sit regularly

Here it simply expresses the action/state of sitting.

Vietnamese usually does not need a separate word like English am before an action verb.

In English:

• I am sitting beside Lan

In Vietnamese:

• Tôi ngồi bên Lan

That is already a complete sentence.

If you want to make the ongoing action clearer, you can add đang:

• Tôi đang ngồi bên Lan. = I am sitting beside Lan.

Bên literally has the idea of side, and in this sentence it means beside / next to / by.

So:

• bên Lan = beside Lan / next to Lan

A couple of similar words are:

• cạnh = next to
• bên cạnh = beside / next to

So these are also natural:

• Tôi ngồi cạnh Lan.
• Tôi ngồi bên cạnh Lan.

This sentence follows a very common Vietnamese pattern:

• Subject + Verb + Location phrase

So here:

• Tôi = subject
• ngồi = verb
• bên Lan = location/position phrase

That makes the sentence structure very straightforward.

Yes. Without extra time markers, Vietnamese often leaves that distinction to context.

So this sentence can mean:

• I sit beside Lan
• I am sitting beside Lan

If you want to be more specific:

• Tôi đang ngồi bên Lan. = I am sitting beside Lan right now
• Tôi đã ngồi bên Lan. = I sat / have sat beside Lan
• Tôi sẽ ngồi bên Lan. = I will sit beside Lan

Vietnamese does not use articles like a, an, and the the same way English does.

Also, Lan is a person's name, so it simply appears on its own:

• Lan

not:

• the Lan
• a Lan

Yes. Lan is a very common Vietnamese given name, often female.

In this sentence, Lan is simply the person’s name, so bên Lan means beside Lan or next to Lan.

It can be figurative in some contexts, because bên can also suggest being on someone’s side.

But in this sentence, because the verb is ngồi (sit), the meaning is almost certainly physical:

• I am sitting next to Lan

So learners should understand it here as a location, not emotional or political support.

A rough English-friendly guide is:

• Tôi: somewhat like toy
• ngồi: starts with ng like the end of sing, then roughly oy
• bên: somewhat like ben, but not exactly the same vowel as English
• Lan: like lahn

A rough pronunciation line would be:

• toy ngoy ben lahn

But Vietnamese tones are very important, so the best way to learn the sentence is by hearing a native speaker say it.