How do you say "I can't do this" in Japanese?

Quick use

Use これはできません (kore wa dekimasen) when you need to say that this is not possible.

Chunk or pattern

• Reusable pattern: 〜 は / できません
• Replace 〜 with the thing that is not possible.

Useful swaps:

• これは できません
• このさぎょうは できません
• きょうは できません

Core words

• これ: this.
• できません: cannot do / is not possible.

Base form contrast

• できます (dekimasu): can do / possible.
• できません (dekimasen): cannot do / not possible.

Practical note

This is direct and useful, but in real work situations you often need to add a reason or an alternative after it.