خواهرم در باغ یک عکس گرفت.

خواهرم is خواهر + -م.

• خواهر = sister
• -م = my

So خواهرم literally means my sister.

Persian very often uses these attached possessive endings:

• -م = my
• -ت = your
• -ش = his/her/its
• -مان = our
• -تان = your
• -شان = their

You can also say خواهرِ من, but خواهرم is shorter and very common in everyday Persian.

در means in, inside, or sometimes at, depending on context.

So:

• در باغ = in the garden / at the garden

Persian does not have a word exactly like English the, so باغ by itself can mean the garden or just a garden, depending on context. In this sentence, English usually translates it as in the garden.

Persian does not have a definite article like English the.

So a bare noun like باغ can be:

• garden
• a garden
• the garden

The context tells you which meaning is intended.

That is very normal in Persian. Learners often expect a separate word for the, but there usually isn't one.

یک literally means one, but it is also commonly used like English a/an.

So:

• یک عکس = a photo / one photo

In this sentence, it mainly works like an indefinite article: a photo.

Persian often uses یک when introducing a singular, indefinite noun.

Yes, it could.

• خواهرم در باغ عکس گرفت.

This is also natural Persian. Depending on context, it can mean something like:

• My sister took pictures in the garden
• My sister took a picture in the garden

Adding یک makes the single photo idea clearer:

• یک عکس گرفت = took one / a photo

Because عکس here is indefinite and non-specific.

In Persian, را usually marks a specific direct object. Since یک عکس means a photo rather than the photo, را is normally not used here.

So these are different:

• یک عکس گرفت = took a photo
• عکس را گرفت = took the photo

That is a very important Persian pattern.

It is a very common Persian expression.

• عکس گرفتن literally looks like to take/grab a picture
• In real usage, it means to take a photo / photograph

So گرفت here comes from گرفتن (to take, seize, get), but together with عکس it functions like the normal verb phrase take a picture in English.

This kind of noun + verb combination is extremely common in Persian.

گرفت is the simple past, third person singular form of گرفتن.

So:

• infinitive: گرفتن = to take
• past stem: گرفت
• گرفت = he/she/it took

Since the subject is خواهرم (my sister), English translates it as my sister took.

Because Persian verbs do not change for grammatical gender.

Persian does not have the kind of masculine/feminine agreement that many learners expect. The verb agrees with the subject in person and number, not in gender.

So گرفت would be the same for:

• خواهرم = my sister
• برادرم = my brother

The noun خواهر itself tells you the person is female, but the verb form does not change because of that.

Because Persian is usually a subject–object–verb language, and the verb often comes at the end.

Here the order is:

• خواهرم = subject
• در باغ = location
• یک عکس = object
• گرفت = verb

So Persian naturally puts the action word last.

This is one of the biggest word-order differences from English.

The given order is natural and correct, but Persian word order can be somewhat flexible.

For example, you may also hear:

• خواهرم یک عکس در باغ گرفت
• در باغ خواهرم یک عکس گرفت

But the most neutral and straightforward pattern is usually to keep the verb at the end. Moving other parts around can change the emphasis or sound more marked depending on context.

So the sentence you have is a very good standard model.

A simple pronunciation guide is:

khāharam dar bāgh yek aks gereft

A few notes:

• خواهرم = khāharam
• باغ = bāgh
  The final gh is a Persian sound written with غ
• عکس = aks
• گرفت = gereft

In normal speech, یک is often pronounced more like ye before a consonant, so you may hear:

khāharam dar bāgh ye aks gereft