又再

Chinese has two words for again: you4 又 and 再 zai4

又 you4 is a highly simplified picture of the right hand, as a symbol of that which recurs, that which happens again and again. Since most people are right handed they use their right hand again and again. The fingers are on the top and left the arm is on the lower right.

You4 又 is used to refer exclusively  to events which have occurred in the past, and only in the past.

你又去机场!  You went to the airport – again! You went once, in the past, and you went yet again, also in the past.

Yesterday
Once AGAIN
U REPEATED IT IN THE PAST

又 is exclusively used for recurring past events, something that happened at least twice in the past.

再

Zai’s
Again
In THE FUTURE

zai is used to refer to the repetition of an event in the future. The characters origins are unclear. It appeared to be a person entering a house, or a person with a hand and tool at a house. It can be decomposed into 工 冂 and 一 which all look like elements from the oracle script. It is probably the idea of a person returning home in the future or working at their home or in a workplace in the future, they will return home AGAIN they will work at the workplace AGAIN but in the future.

再 is used exclusively for things which will occur at least once in the future and have occurred at least once in the past. For example 再见 means literally “see you again”: you see the person now and probably also in the past and plan or at least hope to see them again in the future. Thus 再 and not 又。

You 又 and  zai 再 are examples of context clues to tense intended in a Chinese sentence.