Ofise girer girmez telefonu sessiz moda alıyorum.

It means as soon as (someone) enters. It’s a fixed temporal pattern formed by:

• the aorist form of the verb: gir-er (enter + aorist)
• immediately followed by the negative aorist: gir-mez

So: gir-er gir-mez = as soon as (someone) enters.

More examples:

• gelir gelmez = as soon as (someone) comes
• yapar yapmaz = as soon as (someone) does
• çıkar çıkmaz = as soon as (someone) goes out

The negative part here does not negate the meaning; it’s just the conventional way Turkish expresses “as soon as.”

Because girer girmez is a set temporal construction that uses the aorist, while the main clause’s tense is chosen for the intended meaning. Here alıyorum presents a current/regular habit. You could also say alırım (aorist) to sound more timelessly habitual or general. Both are correct; the nuance differs slightly:

• alıyorum = this is what I typically do these days / in my current routine
• alırım = this is what I (always) do as a rule

Because it’s a definite direct object. Turkish marks definite objects with the accusative:

• telefon = a/any phone (indefinite)
• telefonu = the phone (definite)
• telefonumu = my phone (definite + possessive)

Since you’re putting a specific, context-known phone on silent, Turkish expects the accusative.

Both are possible:

• telefonu = “the phone” (often understood as “my phone” from context)
• telefonumu = explicitly “my phone”

In everyday speech, people often omit the possessive when it’s obvious, but telefonumu is also very natural and a bit clearer.

Literally it’s “to take [something] to silent mode,” but idiomatically it means to set/put something into silent mode. Turkish frequently uses the pattern X’i Y’e almak to mean “put/set X into Y (state).”

Common alternatives:

• telefonu sessize almak = to mute/put the phone on silent (very common)
• uçak moduna almak = to switch to airplane mode

Because -a/-e is the dative, which marks the target state or destination: you are putting the phone “into” silent mode. Modu would be accusative (definite object) and is not correct in this change-of-state construction.

Parallel pattern:

• duvarı maviye boyadım = I painted the wall blue (to blue)
• telefonu sessize/moda aldım = I set the phone to silent/silent mode

Yes, Turkish word order is flexible. Some natural options:

• Ofise girer girmez telefonu sessiz moda alıyorum.
• Telefonu, ofise girer girmez, sessiz moda alıyorum.
• Ofise girer girmez, telefonu sessize alıyorum. (comma optional; see below)

Keep ofise close to girer (they belong together). Girer girmez ofise sounds off.

It’s optional. Many writers put a comma after an initial adverbial clause:

• Ofise girer girmez, telefonu sessiz moda alıyorum. No comma is also acceptable in a short sentence like this.

By default, yes—the subject is understood to be the same as the main clause’s subject. If you want different subjects, make them explicit:

• O ofise girer girmez, ben telefonu sessize alıyorum. = As soon as he/she enters the office, I mute the phone. You can also use -ince/-ınca participles (e.g., o ofise girince) when needed.

You can, but the nuance changes:

• Ofise girince… = when/whenever I enter the office (not necessarily instantly)
• Ofise girer girmez… = the very moment I enter the office (immediacy)

Use a past-tense main clause:

• Ofise girer girmez telefonu sessize aldım. = As soon as I entered the office, I put the phone on silent.

You can also say:

• Ofise girer girmez telefonu sessiz moda aldım.