Sie macht eine Diät.

Not necessarily. Sie is capitalized because it is the first word of the sentence, and in German the first word is always capitalized.

To know what Sie means, you have to look at the verb:

• sie macht = she does / she is doing
• Sie machen = you do / you are doing (formal)

So in Sie macht eine Diät, Sie means she, not formal you, because the verb is macht, not machen.

Because macht is the correct present-tense form of machen for sie meaning she.

The present tense of machen goes like this:

• ich mache
• du machst
• er/sie/es macht
• wir machen
• ihr macht
• sie/Sie machen

Since the subject here is sie = she, the correct form is macht.

Because Diät is a feminine noun: die Diät.

In this sentence, Diät is the direct object of the verb macht, so it is in the accusative case. For feminine nouns, the accusative article is still eine:

• nominative: eine Diät
• accusative: eine Diät

So Sie macht eine Diät is grammatically correct.

It is in the accusative case.

The verb machen takes a direct object, and eine Diät is that object:

• Sie = subject
• macht = verb
• eine Diät = direct object

That is why Diät is accusative here.

This is just a common German expression. Eine Diät machen is an idiomatic way to say that someone is dieting or following a diet.

German often uses machen in expressions where English uses a different verb. So even though machen literally means to make / to do, here the whole phrase eine Diät machen means:

• to go on a diet
• to diet
• to follow a diet

It is a normal and natural phrase in German.

Yes, absolutely.

Both are possible, but there is a small difference in feeling:

• Sie macht eine Diät.
  = She is actively dieting / following a diet plan.

• Sie ist auf Diät.
  = She is on a diet.

The second one sounds a little more like describing her current state, while eine Diät machen can sound a bit more like the action or effort of dieting.

Often, yes, but not always.

In everyday speech, eine Diät machen usually means trying to lose weight by changing what you eat.

But Diät can also refer more generally to a special eating plan, including for health or medical reasons. So the exact meaning depends on context.

The word order is the standard German main-clause pattern:

Subject + verb + object

So here:

• Sie = subject
• macht = finite verb
• eine Diät = object

This is one of the most basic sentence patterns in German.

Yes, that is grammatically possible, but it is less neutral.

German allows different word orders for emphasis, as long as the finite verb stays in the second position in a main clause. So:

• Sie macht eine Diät. = neutral
• Eine Diät macht sie. = emphasizes eine Diät

The second version might sound contrastive, like She is dieting (as opposed to doing something else).

It is pronounced roughly like dee-ET.

A few things to notice:

• The stress is on the second syllable: Di-ÄT
• The ä is pronounced like a short open e sound
• There is a clear break between the two vowel parts

So it does not sound like one smooth syllable. It is distinctly two syllables: Di-ät.

Yes, it is common and easy to understand.

In everyday German, people often use:

• eine Diät machen = to diet
• auf Diät sein = to be on a diet

You may also hear related words such as:

• abnehmen = to lose weight
• Ernährung = nutrition / diet in the general sense of eating habits

So Diät is common, especially when talking about weight loss or a special eating plan.