मरीज वहाँ बैठा है।

Yes:

• मरीज = patient
• वहाँ = there / over there
• बैठा = sitting / seated (masculine singular form)
• है = is

So the Hindi structure is roughly patient there seated is.

Hindi does not have articles like a, an, and the the way English does.

So मरीज can mean:

• a patient
• the patient
• sometimes just patient in a general sense

Which one is meant depends on context. In your sentence, English naturally translates it as the patient.

बैठा is the masculine singular form. It agrees with the person being described.

So:

• मरीज वहाँ बैठा है। = the patient is sitting there. (male patient / masculine agreement)
• मरीज वहाँ बैठी है। = the patient is sitting there. (female patient / feminine agreement)

This is a very common feature of Hindi: words like participles and adjectives often change to match gender and number.

Usually, मरीज stays the same for both male and female patients. The gender is shown by agreement elsewhere in the sentence, such as:

• मरीज बैठा है = the patient is sitting (male)
• मरीज बैठी है = the patient is sitting (female)

So the noun मरीज itself often does not change, but the describing word does.

है is the singular present form of होना, meaning to be.

In this sentence, it helps form the present-state meaning:

• बैठा है = is sitting / is seated

Without है, the sentence would sound incomplete in normal standard Hindi.

Compare:

• मरीज वहाँ बैठा है। = complete sentence
• मरीज वहाँ बैठा। = not normal here as a present-tense statement

These mean different things.

• बैठा है = is sitting / is seated right now
• बैठता है = sits / usually sits / tends to sit

So:

• मरीज वहाँ बैठा है। = the patient is sitting there now
• मरीज वहाँ बैठता है। = the patient sits there / usually sits there

A native English speaker often needs to remember that -ता है is usually habitual, not the normal way to say someone is doing something right now.

They are close, but not exactly the same.

• बैठा है often describes a state: the person is already seated
• बैठ रहा है is the progressive form: the person is sitting / in the process of sitting

In many real situations, both can be translated as is sitting, but the nuance is different.

Examples:

• मरीज वहाँ बैठा है। = the patient is seated there / is sitting there
• मरीज वहाँ बैठ रहा है। = the patient is sitting there / is in the act of sitting there

In everyday Hindi, बैठा है is very natural when someone is already sitting in a place.

Yes. Hindi word order is more flexible than English, though the neutral order here is very natural.

Your sentence:

• मरीज वहाँ बैठा है।

You could also say:

• वहाँ मरीज बैठा है।

That version puts a little more focus on there.

Hindi usually keeps the main verb area near the end, so बैठा है stays together at the end in a normal sentence.

Because मरीज is the subject of an intransitive verb in a present-state construction.

• बैठना = to sit, which is intransitive
• In this kind of sentence, the subject is normally unmarked

So:

• मरीज वहाँ बैठा है। = correct

You do not use को here, and you do not use ने here either.

Very broadly:

• को often marks objects or certain experiencers
• ने is used in specific perfective contexts, mainly with transitive verbs

Neither applies here.

वहाँ is pronounced roughly vahā̃.

Important points:

• the ाँ shows that the final vowel is nasalized
• so it is not just vaha
• it sounds more like vahaan with nasal resonance

It means there / over there, as opposed to:

• यहाँ = here

You may also sometimes see वहां in casual writing, but वहाँ is the standard spelling.

It usually means there, but it often has a slightly distal feel, like over there rather than here.

Hindi commonly contrasts:

• यहाँ = here
• वहाँ = there / over there

So in many contexts, वहाँ suggests a place away from the speaker.