Difference between ないといけない and なくてはいけない

Question

While both mean "I have to / I must (do sth)", I wonder if there is a slight difference in the meaning or politeness level or anything else?

For example, do all these pairs sound exactly the same?

• 時計を直さなくてはいけない vs 時計を直さないといけない
• もう帰らなくてはいけない時間だ vs もう帰らないといけない時間だ
• この部屋は掃除しなくてはいけない vs この部屋は掃除しないといけない

Incidentally, it seems that before ならない, ないと is grammatically wrong; only なくては (or other forms such as なければ) are allowed. Not sure if there's any intuitive explanation for that.

Answer 1

There is no difference in meaning, but in casual conversations, しないといけない is much more common (perhaps simply because it's shorter). しなくてはいけない would sound pompous if used in real informal conversations between friends or family members. しなくては is natural if the speaker is an ojōsama or a stereotypical gentlemen in manga/anime.

しなくては is preferred in business settings, and it's often combined with なりません (a politer version of いけません).

• 時計を直さなくてはなりません。
• もう帰らなくてはならない時間です。