Meaning: Suman vs. Sumanu vs. Sumanai vs. Sumimasen
As I wrote about how Gomenasai, Sumimasen, and Suimasen (all meaning I’m sorry in Japanese) differ in this post,
this time, let’s spot the difference between Sumimasen (すみません), Sumanai (すまない), Sumanu (すまぬ), and Suman (すまん).
Suman vs. Sumimasen in Japanese
We, especially males, use Suman (meaning Sorry) with close friends, but some, even friends, feel the phrase lacks sincerity when the object of apology has serious problems/issues.
So if it is a severe problem, it would be better to use Gomenasai instead of Suman. (Gomenasai would be better (more sincere) than Sumimasen)
Sumanu or Sumanai
In the words of Sumanu and Sumanai, nu (ぬ) and nai (ない) are both negative auxiliary verbs and interchangeable.
However, in nuance, Sumanai sounds more polite than Sumanu. (But the latter is rarely heard in everyday conversation, and we use Suman instead)
As with Suman, males tend to use Sumanai towards friends, but this phrase is also not so polite compared to Sumimasen and is not sincere compared to Gomenasai.
In other words, if the object of apology has serious problems/issues, the phrase may not be appropriate, and Gomenasai would be its better choice.
(Reference Page: Weblio すまぬ )