jiàoliàn zhèngzài yòng pǎobùjī.

正在 marks an action as in progress right now. It is one common way to express the idea of English be doing.

So:

教练正在用跑步机。

means the coach is currently using the treadmill.

A few useful comparisons:

• 教练用跑步机。 = The coach uses / is using the treadmill.
  
  • This is more neutral and does not emphasize the ongoing action as strongly.
• 教练在用跑步机。 = Also very common for is using.
• 教练正在用跑步机。 = Slightly more explicit about right now, in the middle of the action.

用 means to use.

In this sentence, the pattern is:

Subject + 正在 + 用 + object

So:

• 教练 = coach
• 正在 = in the middle of
• 用 = use
• 跑步机 = treadmill

That gives the idea The coach is using the treadmill.

This is natural if you want to talk generally about using that piece of equipment. If you specifically want to say the coach is running on it, Chinese often says:

教练正在跑步机上跑步。

That is more specific than 用跑步机.

Yes, it is natural, but it is a little broad.

教练正在用跑步机。 means the coach is using the treadmill, without saying exactly how.

Depending on context, native speakers might also say:

• 教练正在跑步机上跑步。
  = The coach is running on the treadmill.
• 教练正在用跑步机锻炼。
  = The coach is exercising using the treadmill.
• 教练正在跑步。
  = The coach is running.
  
  • This works if the treadmill part is already understood.

So the original sentence is fine, but there are more specific alternatives if needed.

Mandarin does not normally use a verb like English be to form the progressive.

English says:

• The coach is using the treadmill.

Mandarin instead often says:

• 教练正在用跑步机。

The idea of an ongoing action is shown by 正在 or sometimes 在, not by a separate verb like is.

This is a very important difference between English and Chinese:

• English: be + verb-ing
• Chinese: often 在 / 正在 + verb

Yes. 教练在用跑步机 is perfectly natural.

Both 在 and 正在 can mark an action in progress.

A simple way to think about the difference:

• 在 = progressive, common and neutral
• 正在 = progressive, with a stronger sense of right now / currently in progress

So:

• 教练在用跑步机。
• 教练正在用跑步机。

Both are good. The second one is just a little more emphatic.

Because this sentence uses 用, not a location structure.

Compare these:

• 用跑步机 = use the treadmill
• 在跑步机上 = on the treadmill

So:

• 教练正在用跑步机。
  focuses on using the machine
• 教练正在跑步机上跑步。
  focuses on running on the machine

English often uses on with machines like treadmills, but Chinese can choose either:

• 用 + equipment
• 在 + place/surface + 上 + action

They are related, but not identical.

跑步机 is a compound word:

• 跑步 = to run / running
• 机 = machine

So literally it is something like running machine, which is exactly how Mandarin says treadmill.

This kind of word-building is very common in Chinese. Many nouns are transparent like this.

教练 usually means coach, trainer, or sometimes instructor, depending on context.

For example:

• in sports: coach
• in fitness: trainer
• in driving school: driving instructor

So in this sentence, 教练 could be understood as:

• a sports coach
• a fitness trainer
• another kind of instructor, depending on the situation

Chinese often leaves that kind of detail to context.

The word order is:

Subject + 正在 + Verb + Object

So here:

• 教练 = subject
• 正在 = progressive marker
• 用 = verb
• 跑步机 = object

This is a very common Mandarin pattern.

If you add a time word, it usually comes before 正在:

• 教练现在正在用跑步机。
  The coach is using the treadmill now.

Yes, you can.

教练用跑步机。 is grammatical, but it is less clearly tied to right now. It could mean:

• the coach uses a treadmill
• the coach is using a treadmill

If you want to clearly show an action is happening at this moment, 正在 is very helpful.

So:

• 教练用跑步机。 = more general / context-dependent
• 教练正在用跑步机。 = clearly happening now

Yes. 教练正在用跑步机呢。 sounds natural in conversation.

Sentence-final 呢 can make the ongoing action feel more vivid or more conversational, especially when answering a question like:

• 教练在哪儿？
• 教练正在用跑步机呢。

It is not required, but it is common in speech.

That sounds much less natural here.

For an action actively happening in progress, 在 or 正在 is the normal choice:

• 教练在用跑步机。
• 教练正在用跑步机。

着 often marks a continuing state or a sustained situation, but with 用 in this sentence it does not sound as natural as 在/正在.

So for learners, the safest choice is:

• 在用
• 正在用

No, not in this sentence.

Measure words are needed when you specify a number, for example:

• 一台跑步机 = one treadmill

Here, 跑步机 is just the direct object of the verb 用, so no measure word is needed:

• 教练正在用跑步机。

But if you wanted to say The coach is using a treadmill, you could say:

• 教练正在用一台跑步机。

For machines, 台 is the usual measure word.