أحسنت في العمل.

A common fully vowelled version is:

أَحْسَنْتَ فِي العَمَلِ
if speaking to one male

and

أَحْسَنْتِ فِي العَمَلِ
if speaking to one female

In normal Arabic writing, those short vowels are usually left out, so you often just see:

أحسنت في العمل

أحسنت is a past-tense verb from أَحْسَنَ.

The base verb أحسن can mean things like:

• to do well
• to excel
• to do something in a good way

In this sentence, أحسنت means you did well.

This is very common in Arabic. A past tense verb is often used to praise someone for something they have just done.

So أحسنت literally means you did well, but in actual usage it often works like:

• Well done
• Good job
• You did well

So even though it is grammatically past tense, it can sound very natural as immediate praise.

In Arabic, the subject is often built into the verb itself.

In أحسنتَ / أحسنتِ, the ending tells you that the subject is you.

So Arabic does not need a separate word like أنت here.
The verb itself already means you did well.

You could say أنت أحسنت for emphasis, but normally it is unnecessary.

Without vowel marks, أحسنت could represent both:

• أَحْسَنْتَ = you did well to a male
• أَحْسَنْتِ = you did well to a female

In ordinary unvowelled writing, context usually tells you which one is meant.

If you add the short vowels, the difference becomes visible:

• ـتَ for masculine
• ـتِ for feminine

في usually means in.

In this sentence, it connects the praise to the area or activity involved:

• أحسنت = you did well
• في العمل = in the work / at work / in the task

So في shows the domain in which the person did well.

العمل means the work or the task/work in question.

In Arabic, the definite article الـ is often used where English might be less explicit. Depending on context, العمل can mean:

• the work
• the task
• the job/work
• the activity being discussed

If you said في عمل, that would sound more like in a work/task and is less natural in this kind of praise sentence unless the context specifically calls for an indefinite noun.

Because it comes after the preposition في.

In standard Arabic grammar, a noun after a preposition takes the genitive case, which usually gives it a final kasra sound:

• في العَمَلِ

So the -i ending is there because في is a preposition.

A simple pronunciation guide is:

• aḥsanta fī l-ʿamali for a male
• aḥsanti fī l-ʿamali for a female

A few notes:

• ح is a strong breathy h sound, not exactly like normal English h
• ع in العمل is a consonant English does not have; learners often approximate it at first
• في has a long ī sound, like fee

Yes, very commonly.

أحسنت! by itself is a standard way to say:

• Well done!
• Good job!
• Excellent!

Adding في العمل makes the praise more specific by saying what the person did well in.

It is a verbal sentence, because it begins with a verb:

• أحسنت = verb
• في العمل = prepositional phrase

Arabic often begins sentences with a verb, especially when describing an action or giving a reaction like praise.

Yes, but the structure and nuance are a little different.

• أحسنتَ في العملِ = You did well in the work/task
• أحسنتَ العملَ can mean something like You did the work well or You performed the task well

The version with في is very common when praising someone in a general way for their performance in some area. The direct-object version is possible too, but it is a slightly different construction.

It is related to the root ح س ن, which carries the idea of goodness, beauty, or being good.

From the same root, you may also see words like:

• حَسَن = good / حسن as a name
• حُسْن = beauty, goodness
• إحسان = excellence, doing good, beneficence

So أحسن is part of a larger family of words connected with goodness and excellence.