…的【de】

的 at the end of a sentence makes it a firm assurance: this is how it is, or how it will be. It most often closes a sentence with 会 or 能, and it calms or reassures the listener.

Without 的, 他会来 is a plain prediction. With it, the speaker is vouching for it.

Without 会

The same 的 can close an adjective or a judgement, with the same "believe me" force.

Where it goes wrong

A promise about the future takes 的, not 了. 了 would announce that something has already changed.

• × 别担心，他会来了 → 别担心，他会来的

It is easy to confuse with 是…的, which picks out when, where or how something already happened: 我是坐飞机来的. The assertive 的 needs no 是, and it is usually about now or the future.