〜ていく

About

The ～ていく structure is comprised of a て form verb + 行いく ("to go"). ～ていく describes something that moves physically or conceptually away from a given current position in either time OR space.

E.g. 鳥とりが飛とんでいった。
"The bird flew away."

Usually, when ～ていく describes something temporally, it describes a gradual change or an action that happens over an extended period of time.

E.g. これから観光客かんこうきゃくが増ふえていくと思おもう。
"I think the number of tourists will start to increase now."

Summary

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  ～ていく describes something that moves physically/conceptually away from a given current position in either time OR space

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  When used temporally, ～ていく tends to describe a gradual action