nà wèi lǎo xiānsheng zài jiāoshuǐ.

位 is a measure word (also called a classifier) for people.

In Chinese, when you say this/that + noun, you usually need a measure word in between:

• 这位老师 = this teacher
• 那位老先生 = that elderly gentleman

For people, 位 is a polite, respectful classifier. That fits well here because the sentence is talking about an older man respectfully.

Yes, you could.

• 那位老先生 sounds more polite and respectful.
• 那个老先生 is more neutral and everyday.

For an older person, 位 is often the better choice if you want to sound courteous.

No, it is not rude here.

In Chinese, 老 does not always work like English old. It can suggest:

• someone elderly
• familiarity
• respect, depending on context

So 老先生 is usually understood as elderly gentleman or old gentleman in a respectful way, not an insult like English old man can sometimes be.

Chinese noun phrases often follow this pattern:

demonstrative + measure word + noun

So here:

• 那 = that
• 位 = classifier for people
• 老先生 = elderly gentleman

Together, 那位老先生 means that elderly gentleman.

Notice that Chinese usually does not say it in the English-style order of that old gentleman with no classifier. The classifier is an important part of the structure.

Here, 在 marks an action as in progress.

So:

• 浇水 = water / do watering
• 在浇水 = be watering, be in the middle of watering

That means this sentence describes what the man is doing right now.

This is different from 在 meaning at/in/on a place. In this sentence, 在 is not showing location; it is showing an ongoing action.

Yes, but the meaning changes a little.

• 那位老先生在浇水。 = That elderly gentleman is watering.
• 那位老先生浇水。 = That elderly gentleman waters / is watering.

Without 在, the sentence is less clearly focused on the action happening right now. It can sound more general or habitual, though context can still make it mean a current action.

If you want to emphasize even more strongly that it is happening right now, you can also use 正在:

• 那位老先生正在浇水。

It is a common verb expression meaning to water.

Literally:

• 浇 = pour/sprinkle
• 水 = water

But together 浇水 is a standard way to say water plants / do watering.

Chinese often uses expressions like this where the literal parts do not map perfectly to a single English word, but the full expression is very natural.

Chinese often leaves out an object if it is obvious from context.

So 浇水 by itself can mean:

• watering the plants
• watering the garden
• watering something nearby that both speaker and listener understand

If you want to say exactly what is being watered, you can add it:

• 给花浇水 = water the flowers
• 给植物浇水 = water the plants

So the sentence does not need an object if the situation already makes it clear.

The location usually goes after the subject and before the main verb.

For example:

• 那位老先生在院子里浇水。 = That elderly gentleman is watering in the yard.

Be careful: here 在院子里 means in the yard, so 在 is marking location.

If you want both a clear progressive meaning and a location, 正在 is often very helpful:

• 那位老先生正在院子里浇水。

That makes the sentence easier to understand as He is currently watering in the yard.

Yes, in modern Mandarin 先生 is very often pronounced xiānsheng, with the second syllable in a neutral tone.

So it sounds like:

• xiānsheng

rather than a fully stressed xiānshēng in normal speech.

This is common in everyday pronunciation. Learners often notice this because the written pinyin may look like two full syllables, but the second one is usually lighter in actual speech.

Yes, but it sounds a bit respectful and sometimes slightly formal or traditional.

You may hear:

• 先生 as a polite title, especially after a surname
• 老先生 when referring respectfully to an elderly man

So this sentence sounds natural, especially if the speaker is being polite or respectful toward the man.