wéixiūrényuán zhídào jiǎnchá le fámén, cái gēnghuàn sǔnhuài de língjiàn.

It means not until... did... or only after... did....

In this sentence, 直到检查了阀门，才更换损坏的零件 shows that the maintenance personnel waited until the valve had been inspected before replacing the damaged part(s).

So it is stronger than a simple sequence like after inspecting the valve, they replaced the parts. It emphasizes that the replacement did not happen earlier.

才 adds the idea of only then or not until then.

Without 才, the sentence would lose some of its emphasis. Compare:

• 检查了阀门后，更换了损坏的零件。
  This mainly says one thing happened after another.

• 直到检查了阀门，才更换损坏的零件。
  This stresses that the replacement happened only after the inspection.

So 才 is an important part of the delayed-action meaning.

The 了 after 检查 is an aspect particle showing that the inspection was completed.

Here, that completion matters because the sentence is saying the second action happened only after the first one was finished.

So:

• 检查了阀门 = inspected the valve / completed the inspection of the valve

It helps mark the inspection as a finished event before the replacement.

Chinese does not always need 了 on every completed action.

In this sentence, 更换 can stay without 了 because the structure 直到...才... already clearly presents the sequence of events, and the sentence can be understood as a factual statement.

You could also say:

• 维修人员直到检查了阀门，才更换了损坏的零件。

That version is also natural, and it makes the replacement sound more explicitly completed.

So the version without 了 is not wrong at all; it is just a little less explicit about completion.

By itself, 维修人员 can be singular or plural. Chinese nouns usually do not show number the way English nouns do.

So depending on context, it could mean:

• the maintenance worker
• the maintenance personnel
• the maintenance staff

If the speaker wants to make plurality clearer, they could add context or a number, such as 一名维修人员 for one person or 几名维修人员 for several.

Yes, the sentence strongly suggests that they are the same people.

The subject 维修人员 appears at the beginning and applies to both actions:

• 检查了阀门
• 更换损坏的零件

Chinese often does not repeat the subject when it stays the same. So this is very natural.

This is a very common Chinese modifier pattern:

modifier + 的 + noun

Here:

• 损坏 = damaged / to be damaged
• 的 = links the modifier to the noun
• 零件 = part / component

So:

• 损坏的零件 = the damaged part(s)

In English, we usually put the modifier before the noun directly, but in Chinese, 的 often connects them.

It behaves like a verb-origin modifier here.

Basically, 损坏 originally means to damage / to be damaged, but in 损坏的零件, it describes the condition of 零件, so it works like damaged in English.

This is very common in Chinese: a verb or verb phrase can come before 的 to describe a noun.

Yes. 直到 can be followed by:

• a time: 直到今天
• an event or action: 直到检查了阀门

In this sentence, it is followed by an action phrase, and that action acts like the boundary point: only after the valve inspection.

So 直到 does not have to mean only until a clock time. It can also mean until a certain event happened.

Yes, for example:

• 维修人员检查了阀门后，才更换损坏的零件。
• 维修人员在检查了阀门以后，才更换损坏的零件。

These are natural too.

But there is a nuance:

• 检查了阀门后... mainly says after inspecting the valve...
• 直到检查了阀门，才... emphasizes not until the valve was inspected...

So the original sentence has a stronger sense of delay or restraint before the second action.

The comma separates the until-clause from the main action:

• 直到检查了阀门，
• 才更换损坏的零件。

It helps the reader hear the sentence structure clearly. In Chinese, commas are often used to divide clauses more freely than in English, especially when a sentence has a clear two-part pattern like 直到...才....