Yeni bir dilbilgisi konusu olarak “-den beri” ekini tanıtıyoruz; bu ek, bir eylemin yapıldığı zamandan itibaren hâlâ devam ettiğini anlatır.

-den beri literally means “since (that time)” and tells you that an action or state that began at a specific point in the past is still continuing now. You attach it to a time expression to mark the starting point.
Example:
Sabah kahvaltıdan beri kahve içiyorum.
(“I have been drinking coffee since breakfast.”)

1. First add the ablative suffix -den to your noun; then write beri as a separate word.
2. Follow Turkish vowel harmony and consonant assimilation rules:
   
   • If the last sound of the noun is a front vowel (e, i, ö, ü), use -den.
   • If it’s a back vowel (a, ı, o, u), use -dan.
   • After a voiceless consonant you often see a t instead of d (kitap → kitaptan beri).
3. Always separate beri with a space: e.g. sabah
   • -tan
     • beri → sabahtan beri.

Most commonly you see it with the present continuous (–yor) to emphasize an unbroken action or state:
Üç saattir bekliyorum.
You can also use it with simple present when describing a continuing state or habit:
On yıldır bu şehirde yaşıyorum.

Yes. You simply negate or question the verb as usual, while the time expression stays the same:
Negative: Dün geceden beri uyumadım. (“I haven’t slept since last night.”)
Question: Ne zamandan beri Türkçe öğreniyorsun? (“Since when have you been learning Turkish?”)

They are virtually interchangeable:
İki yıldır bu işi yapıyorum.
İki yıldan bu yana bu işi yapıyorum.
İki yıldan beri bu işi yapıyorum.
All mean “I have been doing this job for two years,” though -den beri and -dan bu yana are more focused on the starting point (“since two years ago”).

• iki yıldır is a single suffix (-dır) attached to the time noun, meaning “for two years.”
• iki yıldan beri spells out the ablative (-dan) plus beri, literally “since two years [ago].”
  Meaning-wise they overlap a lot, but -den beri emphasizes the moment something started, whereas -dır simply states the duration.

Yes. You can attach it to:
• Numbers + time units: üç günden beri (“since three days ago”)
• Named days/months: Haziran’dan beri (“since June”)
• Specific events: sınavdan beri (“since the exam”)

Use a question word with -den beri:
Ne zamandan beri …? — Since when …?
Ne zamandan beri sinemaya gitmiyorsun? (“Since when haven’t you been going to the cinema?”)
You can also specify:
Hangi tarihten beri bu şirkette çalışıyorsun? (“Since which date have you been working at this company?””)