How does the suffix -dir work.

I have seen attached to many words, such as iyidir, süredir etc.

Thank you

please someone correct me if I am wrong because I am only learning as well but doesn´t 

dir/dır/dur/dür mean ´is´

For example: iyidir - is good?

The two major uses I am aware of are

1) added after a noun or adjective to mean (he/she/it) is (or are).

Türktür = He/she is Turkish

Pırlanta pahalıdır = Diamonds are expensive.

Yeşildir = It is green.

2) added after words used to mean a period of time (day, week, hour) to mean for. (similar in usage to the dan beri suffix) 

Note: the action should still be continuing now, and this form is not used for an action that has ended.

Dört yıldır Türkçe öğreniyorum. = I have been learnining Turkish for 4 years (and I am still learning now).

Telefon üç gündür çalışmıyor. = The phone has not been working for three days. (and is still not working)

On haftadır evliyiz. = We have been married for 10 weeks. (and we are still married)

İki aydır İstanbul´da yaşıyorum. = I have been living in Istanbul for 2 months. (and I am still living there.)

For süredir and zamandır look at Tunci´s posts when you click on the blue words.

great explanation. 

henry thank you for your exposition