Questions & Answers about Sá sem kemur seint þarf að bíða lengur.
Sá sem is a very common Icelandic pattern meaning the one who, he who, or more naturally in English, whoever.
- sá = that one / he
- sem = a relative word like who / that
So Sá sem kemur seint literally means the one who comes late.
In sentences like this, sá sem is often used in a general sense, not necessarily about one specific male person. It is a standard way to introduce a rule or general statement.
Kemur is the 3rd person singular present form of koma (to come).
The subject is Sá sem kemur seint = the one who comes late, which is treated as singular, so the verb must also be singular:
- ég kem = I come
- þú kemur = you come
- hann/hún/það kemur = he/she/it comes
Here, the person is not named directly, but sá functions like a singular subject, so kemur is the correct form.