Boşuna endişelenme; anahtar çantamdaymış.

Boşuna means for nothing, in vain, or needlessly.

So Boşuna endişelenme means something like:

• Don’t worry for nothing
• Don’t worry unnecessarily
• There’s no need to worry

It often gives the idea that the worry was unnecessary because the problem is already solved or was never really a problem.

Because endişelenme is the negative imperative of endişelenmek, which means to worry or to become anxious.

Breakdown:

• endişelen- = worry / become anxious
• -me = negative imperative for informal singular you

So:

• Endişelen. = Worry.
• Endişelenme. = Don’t worry.

In this sentence, it is talking to one person in an informal way.

Yes. Endişelenme is the informal singular command, used when speaking to one person you would address as sen.

For plural or polite you, Turkish uses:

• Endişelenmeyin. = Don’t worry. (plural or polite)

So:

• Boşuna endişelenme. = to one person informally
• Boşuna endişelenmeyin. = to several people, or politely to one person

Turkish often leaves out subject pronouns when they are already clear from the verb form.

In an imperative, the subject is naturally understood:

• Endişelenme already means don’t you worry

You could add sen for emphasis, but normally you would not:

• Sen boşuna endişelenme. = You don’t worry unnecessarily.

That sounds more emphatic than the basic sentence.

Çantamdaymış can be broken down like this:

• çanta = bag
• -m = my
• -da = in / at / on
• -y- = buffer consonant
• -mış = evidential/reported/discovered past marker

So:

• çantamda = in my bag
• çantamdaymış = apparently it was in my bag / turns out it was in my bag

The -y- appears because -mış is being added after a vowel.

This is one of the most important parts of the sentence.

The suffix -mış / -miş / -muş / -müş often shows that the speaker:

• learned the information later,
• is reporting it,
• inferred it,
• or has just realized it

So anahtar çantamdaymış does not sound like a plain neutral fact. It sounds more like:

• Apparently the key was in my bag.
• It turns out the key was in my bag.
• Oh, the key was in my bag after all.

It can also carry a sense of surprise or discovery.

Compare:

• Anahtar çantamdaydı. = The key was in my bag.
  Neutral statement of fact.

• Anahtar çantamdaymış. = Turns out the key was in my bag.
  Newly discovered / inferred / reported information.

Because Turkish often does not use a separate word like English to be in simple noun or location sentences.

For example:

• Anahtar çantamda. = The key is in my bag.

There is no separate word for is. The location phrase itself works as the predicate.

Then Turkish adds tense or evidential meaning as a suffix:

• çantamda = in my bag
• çantamdaydı = was in my bag
• çantamdaymış = was apparently / turns out was in my bag

So the be idea is built into the structure rather than expressed as a separate verb.

Because anahtar here is the subject, not a direct object.

• Anahtar çantamdaymış. = The key was in my bag.

The ending -ı / -i / -u / -ü often marks a definite direct object, not a subject.

For example:

• Anahtarı buldum. = I found the key.
  Here anahtarı is the object.

But in your sentence, anahtar is the thing being talked about, so plain anahtar is correct.

Also, Turkish does not have articles like a/an/the, so anahtar can mean:

• a key
• the key

The context tells you which one is meant.

Yes, very often you could.

• Merak etme is a very common everyday way to say don’t worry
• Endişelenme is also correct, but it sounds a bit closer to don’t be anxious or don’t be concerned

So the difference is mostly one of tone:

• Merak etme = very common, conversational
• Endişelenme = slightly more specific to anxiety/concern

In this sentence, Boşuna merak etme; anahtar çantamdaymış. would also sound natural.

The semicolon links the two parts closely:

• Boşuna endişelenme
• anahtar çantamdaymış

The second part explains why the speaker says not to worry.

In English, this is like:

• Don’t worry unnecessarily; the key was in my bag after all.

In Turkish, people might also write this with:

• a comma
• a period
• or even just say it with a pause in speech

So the semicolon is not the main grammar point here; it mainly shows a clear pause and a strong connection between the two ideas.

This word order is very natural in Turkish.

• Anahtar = the topic / thing being talked about
• çantamdaymış = the main new information

So it feels like:

• As for the key, it turns out it was in my bag.

Turkish word order is flexible, but changes emphasis. For example:

• Anahtar çantamdaymış. = neutral and natural
• Çantamdaymış anahtar. = more marked, often poetic or strongly contrastive

So the original version is the standard everyday choice.