Learn Turkish: Dışarıdan çevrimiçi sisteme girilmez. - One doesn't enter the online system from outside.

In Turkish, you can add the passive suffix to an intransitive verb (a verb that does not take a direct object) to create an impersonal meaning. Instead of saying someone is entered, girilmez translates to 'one does not enter' or 'people generally do not enter'. It is commonly used for general rules or public signs.

Why use the aorist negative -mez here instead of the present continuous girilmiyor?

The aorist tense is used because this states a general rule, policy, or constant fact. If you used girilmiyor, it would sound like a temporary technical issue or a specific action that just isn't happening right at this moment.

Why is it sisteme with the dative ending instead of the accusative sistemi?

In English, you 'enter a system' directly. However, the Turkish verb girmek (to enter) requires a destination, so it always points towards something using the dative case (-e or -a). You are literally saying 'enter to the system'.

How is the word çevrimiçi formed to mean 'online'?

It is a compound word made of çevrim (cycle or circuit) and içi (inside of). Literally meaning 'inside the circuit', it is the standard Turkish word for 'online'. As a bonus, the opposite 'offline' is çevrimdışı (outside the circuit).