Για το γραφείο θέλω πουκάμισο που να μην είναι ούτε ριγέ ούτε καρό, αλλά απλό και καθαρό.

Yes. Για το γραφείο literally means for the office.

In this sentence, it means something like:

• for office wear
• for work
• to wear at the office

Greek often uses για + accusative to show purpose or intended use. So για το γραφείο tells you what kind of shirt the speaker wants: one suitable for the office.

A very natural English equivalent is not always the literal for the office, but that is the basic structure.

Both are possible.

• θέλω πουκάμισο = I want a shirt / I want some kind of shirt
• θέλω ένα πουκάμισο = I want one shirt / a shirt

Greek often leaves out the indefinite article ένα when speaking generally or when the exact item is not yet identified. Without ένα, the sentence can sound a little more focused on the type of item wanted rather than on counting it.

So here:

• θέλω πουκάμισο sounds natural and idiomatic
• θέλω ένα πουκάμισο would also be correct

Here που introduces a relative clause, meaning that or which:

• πουκάμισο που... = a shirt that...

So:

• θέλω πουκάμισο που... = I want a shirt that...

What follows is the description of the shirt the speaker wants.

In Greek, που is very commonly used as a relative word for that / which / who.

This is a very important pattern.

• που δεν είναι... would describe an already known, existing shirt: a shirt that is not...
• που να μην είναι... describes the kind of shirt the speaker wants to find: a shirt that should not be... / that isn’t to be...

After words like θέλω, Greek often uses:

• noun + που να + verb

to express a desired characteristic.

So:

• Θέλω πουκάμισο που να μην είναι ριγέ
  = I want a shirt that isn’t striped

This does not point to one specific shirt already identified. It describes the qualities the wanted shirt should have.

Because να normally takes the negative μη(ν), not δεν.

Compare:

• Δεν είναι ριγέ. = It is not striped.
• Να μην είναι ριγέ. = for it not to be striped / that it not be striped

So:

• δεν is used with ordinary indicative statements
• μην is used with να clauses and other non-indicative contexts

That is why που να μην είναι is correct.

It is a subjunctive construction with να.

More specifically:

• είναι is the present form of είμαι
• with να, it functions as a present subjunctive-type form

In practical learner terms, you can think of it as:

• να είναι = to be / should be
• να μην είναι = not to be / should not be

Here it expresses a general desired quality, not a completed action.

Ούτε ... ούτε ... means neither ... nor ...

So:

• ούτε ριγέ ούτε καρό = neither striped nor checked

This is the standard Greek way to join two negative alternatives.

Examples:

• Δεν θέλω ούτε καφέ ούτε τσάι. = I want neither coffee nor tea.
• Δεν είναι ούτε ακριβό ούτε φτηνό. = It’s neither expensive nor cheap.

In your sentence, the shirt should be:

• not striped
• and not checked

They are clothing-pattern adjectives:

• ριγέ = striped
• καρό = checked / plaid

These words are commonly treated as indeclinable, meaning their form usually stays the same instead of changing for gender, number, or case.

So you can get things like:

• ριγέ πουκάμισο
• ριγέ φούστα
• καρό σακάκι
• καρό παντελόνι

The noun changes as needed, but ριγέ and καρό usually stay the same.

Because they agree with πουκάμισο, which is:

• neuter
• singular
• accusative here, though for neuter singular the form is the same as nominative

So the adjectives must match:

• πουκάμισο → neuter singular
• απλό
• καθαρό

Compare with other genders:

• απλός / καθαρός for masculine singular
• απλή / καθαρή for feminine singular
• απλό / καθαρό for neuter singular

So the endings are there because of adjective agreement.

In this context:

• απλό means plain, simple, not flashy or patterned
• καθαρό means clean

So the shirt should be:

• not striped
• not checked
• plain
• clean

In clothing descriptions, απλό often contrasts with patterned, decorative, or busy-looking clothes.

Here αλλά simply means but.

It contrasts two sets of qualities:

• what the shirt should not be: ούτε ριγέ ούτε καρό
• what it should be: απλό και καθαρό

So the structure is:

• not this, not that, but this and that

That is exactly how αλλά works here.

Yes. Greek word order is flexible, and several versions would be natural.

For example:

• Για το γραφείο θέλω πουκάμισο...
• Θέλω πουκάμισο για το γραφείο...
• Θέλω ένα πουκάμισο για το γραφείο...

Starting with Για το γραφείο gives that part a little emphasis, as if the speaker is setting the context first:

• As for the office, I want a shirt...

So the original word order is natural and slightly highlights the purpose: office use.

Yes, there are other ways to express a similar idea, but the original sentence is very natural.

Possible alternatives include:

• Θέλω ένα απλό, καθαρό πουκάμισο για το γραφείο, όχι ριγέ ή καρό.
• Θέλω πουκάμισο για το γραφείο χωρίς ρίγες ή καρό σχέδια.

However, που να μην είναι... is a very useful and common structure because it neatly expresses the desired characteristics of something not yet chosen.

So even if there are simpler rewrites, the original is worth learning as a pattern:

• θέλω + noun + που να + verb