من کلید را گم کرده ام، اما خواهرم آن را پیدا کرده است.

Yes, it can often be left out.

In Persian, the verb ending usually shows the subject clearly:

• کرده‌ام = I have done
• کرده است = he/she/it has done

So من کلید را گم کرده‌ام is grammatically fine, but کلید را گم کرده‌ام is also completely natural and often more common.

Using من adds emphasis or contrast, something like:

• I have lost the key, but my sister has found it.

So in this sentence, من helps set up the contrast with خواهرم.

را marks a specific direct object.

So:

• کلید را = the key as the object of the verb
• not just any key, but a particular key

In this sentence, کلید is the thing that was lost, so Persian marks it with را.

A very common way to think about را is:

• it often appears when English would have the, this, that, it, or some other definite/specific object

So:

• کلید را گم کرده‌ام = I have lost the key
• آن را پیدا کرده است = she has found it / that

For the same reason: آن is also the direct object of the verb پیدا کرده است.

Here:

• آن = that / it
• آن را = it as a marked direct object

So the structure is:

• خواهرم آن را پیدا کرده است
• My sister has found it.

Persian often uses آن را where English just says it.

Literally, آن often means that, but in sentences like this it commonly works like it.

Because کلید was already mentioned, آن refers back to it. So in natural English, you translate it as it, not that.

So:

• آن را پیدا کرده است = has found it

This is very normal in Persian.

The -م at the end is a possessive ending meaning my.

So:

• خواهر = sister
• خواهرم = my sister

This is one of the most common Persian patterns:

• کتابم = my book
• دوستم = my friend
• برادرم = my brother

So instead of using a separate word for my, Persian often attaches it to the noun.

These are in the present perfect.

Structure:

• past participle
  • present form of to be

Examples here:

• کرده‌ام = have done
• کرده است = has done

With the full expressions:

• گم کرده‌ام = have lost
• پیدا کرده است = has found

This is very close to English present perfect in form, though the exact usage can differ somewhat depending on context.

Good question. In Persian, many actions are built with a noun/adjective-like element plus کردن (to do).

Here:

• گم کردن = to lose
• پیدا کردن = to find

So the meaningful part is:

• گم = lost / missing
• پیدا = found / visible / discovered

And Persian turns them into full verbs with کردن.

That is why in the present perfect you see:

• گم کرده‌ام
• پیدا کرده است

You are really seeing the present perfect of گم کردن and پیدا کردن.

It functions as one verbal unit, but it is often written as two parts:

• کرده = done
• ام / است = am / is

Together they make the perfect tense.

In careful modern spelling, you may also see:

• کرده‌ام
• کرده‌است

with a half-space or joiner.

So these are all related ways of writing the same structure. For learners, the important point is that:

• کرده‌ام = have done
• کرده است = has done

Even if it looks split on the page, you should understand it as a single tense form.

Persian usually follows Subject – Object – Verb order.

So:

• من = subject
• کلید را = object
• گم کرده‌ام = verb

That gives:

• من کلید را گم کرده‌ام

Literally, something like:

• I the key lost have

The second clause works the same way:

• خواهرم آن را پیدا کرده است
• my sister it found has

This verb-final pattern is one of the biggest differences from English.

اما means but / however.

It connects the two clauses and shows contrast:

• I have lost the key, but my sister has found it.

It is a fairly standard written and spoken word. In everyday speech, many speakers also use ولی for but.

So both of these are common:

• اما
• ولی

Yes, it could, but using آن را is more natural once the object has already been mentioned.

Compare:

• من کلید را گم کرده‌ام، اما خواهرم آن را پیدا کرده است.
• I have lost the key, but my sister has found it.

Versus repeating the noun:

• من کلید را گم کرده‌ام، اما خواهرم کلید را پیدا کرده است.

The second version is understandable, but it sounds more repetitive, just like repeating the key in English would.

Yes, very often.

A very natural version would be:

• کلید را گم کرده‌ام، اما خواهرم آن را پیدا کرده است.

And if context is very clear, Persian often omits even more. Subject pronouns like من are commonly dropped unless they are needed for emphasis, contrast, or clarity.

In this sentence, من is optional, but خواهرم is not, because you need to know who found it.

Yes.

• گم کرده‌ام = I have lost
• گم کردم = I lost

The first is present perfect; the second is simple past.

In many real conversations, Persian does not always match English tense usage exactly, and sometimes the simple past is used where English might prefer a present perfect. But as a basic distinction:

• گم کرده‌ام often emphasizes the current result: the key is lost now
• گم کردم simply reports the past event

So in this sentence, گم کرده‌ام fits well because the result still matters: the key was lost, but now the sister has found it.

In everyday speech, pronunciation is often a bit more compressed.

For example:

• کرده‌ام may sound closer to kardam / karde-am depending on accent and speed
• است is often pronounced more lightly, and in informal speech people may use ه instead:
  • پیدا کرده است → پیدا کرده‌ه or more naturally پیدا کرده

A colloquial version might be:

• کلیدو گم کردم، ولی خواهرم پیداش کرده.

That is much more conversational. But the sentence you were given is a good standard written form, and it is absolutely worth learning first.