(This is a screenshot of opening of chapter 20 of Shin Nihongo No Kiso book. This is done just for the short recap of what I learned in this chapter.)
1. Ashita tokyo e iku. Tomorrow, I will go to Tokyo.
The ku-form of the verb is used when you want to express in informal way. It is often used between families and friends. Like for this example. Iku is the ku-form of ikimasu which is more polite and formal. Their meaning is just the same.
2.Mainichi isogashii. I am busy everyday.
When expressing informality for adjectives, it should be in the base form. Just like in the example above. If we’re going to show formality, we should have said Mainichi isogashii desu. If we’re going to put it in negative, meaning we’re going to say “not busy” it would be “Mainichi isogashikunai.” In formal form it should be, Mainishi isogashiku arimasen. In other words, the informal form of arimasen is “nai”.
3.Kyou wa ii tenki da. The weather today is good.
Here, it shows that the informal form of desu is da. To put the sentence in formal form, it would be “Kyouwa ii tenki desu.”