wǒ yǐjīng wánchéng gōngzuò.

已经 is an adverb meaning already. It tells you that the action was completed before now or earlier than expected.

In this sentence, the basic structure is:

我 + 已经 + 完成 + 工作
subject + already + complete + work

So 已经 goes before the verb phrase, not after it.

A lot of learners expect 已经 and 了 to appear together, because that is very common:

我已经完成工作了。
我已经完成了工作。

Both are natural.

But 了 is not required every time. In 我已经完成工作。, the combination of 已经 and 完成 already makes the idea of completion very clear. The sentence sounds a bit more neutral or formal without 了.

So:

• 我已经完成工作。 = correct, somewhat concise/formal
• 我已经完成工作了。 = very common, often more conversational
• 我已经完成了工作。 = also very common

The pattern is:

Subject + Adverb + Verb + Object

In this case:

• 我 = subject
• 已经 = adverb
• 完成 = verb
• 工作 = object

So the word order is very typical Mandarin word order.

Here, 工作 is a noun, meaning work, task, or job-related work.

That is because it comes after 完成, and 完成 needs an object:

• 完成工作 = complete the work

But 工作 can also be a verb in other sentences:

• 我工作。 = I work.

So 工作 can be either a noun or a verb depending on how it is used.

Because 完成 is the verb and 工作 is its object.

In Mandarin, verbs normally come before their objects:

• 完成工作 = complete the work
• 吃饭 = eat food / eat a meal
• 写作业 = do homework

If you say 工作完成, that usually sounds like the work is completed, where 工作 is now the topic or subject, not the object of the verb. So it is a different structure.

完成 means to complete, to accomplish, or to finish something, often with a slightly more formal tone.

做完 means to finish doing something and is often more everyday and conversational.

Compare:

• 我已经完成工作。 = a bit more formal; I have already completed the work.
• 我已经做完工作了。 = more conversational; I’ve already finished the work.

In work, school, or business contexts, 完成 is very common.

Not necessarily.

工作 by itself can mean work in a general sense or the work when the context is clear.

So 我已经完成工作。 is understandable as I have already completed the work or I have finished my work, depending on context.

If you want to be more specific, you can say:

• 我已经完成我的工作了。 = I have already finished my work.
• 我已经完成这项工作了。 = I have already completed this task.

So leaving out 我的 is often fine if the listener already knows which work you mean.

Sometimes, but not always.

我已经完成工作。 most directly means I have already completed the work. It focuses on finishing a task or assignment.

If you want to say I’m done with work for today or I’ve gotten off work, more natural choices might be:

• 我已经下班了。 = I’m already off work.
• 我今天的工作已经完成了。 = I’ve already finished today’s work.

So the exact meaning depends on context.

Yes. That is very natural.

These are all possible:

• 我已经完成工作。
• 我已经完成了工作。
• 我已经完成工作了。

Very roughly:

• 完成工作: neutral and concise
• 完成了工作: clearly marks the action as completed
• 完成工作了: often sounds more like reporting a new situation or completed state

For a learner, both 我已经完成了工作 and 我已经完成工作了 are very safe, natural choices.

It sounds a bit more formal or written because of 完成 and because there is no 了.

In everyday speech, people might more often say:

• 我已经做完了。
• 我已经把工作做完了。
• 我工作已经做完了。

But 我已经完成工作。 is still correct and natural, especially in formal speech, writing, work reports, or business contexts.