[文法] Why does it use ing form here?

[jeffguoft (見習生_O7)]

"Switching gears for a second"
Could someone tell me why does it use ing form in the sentence, not verb?

If I want to express it's the second day I start to practice English,
Should I say "Practing English on day 2" or "Practice English on day 2"
or other ways?

and if there are some other mistakes in grammar in the post please tell me.
I want to prove my grammar.

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※ 編輯: jeffguoft (111.254.18.171), 05/27/2018 08:18:09
推 leo97728: Day two of practicing English
→ leo97728: Practicing English on day two, means on day 2 of "so
→ leo97728: mething",  (ex. the summer holidays) your choice of
→ leo97728: activity is to practice English.