ニヤつく

Different conjugations of the verb in both plain form and keigo (polite)

Verb conjugations

1. Present tense (used to describe a current action) - 現在形（げんざいけい）
2. Present negative tense (used to describe a current action, but with a negative meaning) - 現在否定形（げんざいひていけい）
3. Past tense (used to describe a past action) - 過去形（かこけい）
4. Past negative tense (used to describe a past action, but with a negative meaning) - 過去否定形（かこひていけい）
5. Te form (used for connecting verbs or for making requests) - て形（てけい）
6. Tai form (used to express desire to do something) - たい形（たいけい）
7. Volitional form (used to express intention or determination) - 意向形（いこうけい）
8. Imperative form (used to give commands) - 命令形（めいれいけい）
9. Passive form (used to describe an action being done to the subject) - 受身形（うけみけい）
10. Conditional form (used to describe a hypothetical situation) - 条件形（じょうけんけい）
11. Provisional conditional form (used to describe a tentative or uncertain situation) - 定形（かていけい）
12. Causative form (used to describe causing someone to do something) - 使役形（しえきけい）
13. Potential form (used to describe ability to do something) - 可能形（かのうけい）

Plain form vs keigo

Plain form is the basic form of a verb or adjective in Japanese. It's called "plain" because it's the simplest and most straightforward form, without any added politeness or formality. Plain form is used in informal situations, such as talking to friends, family, or people of lower status.

Keigo, on the other hand, is a set of honorific language expressions used to show respect, politeness, and formality in Japanese.

In general, plain form is used in informal situations and keigo is used in formal situations. However, it's important to note that there are different levels of keigo depending on the level of formality and the relationship between the speaker and listener. For example, the keigo used with a superior in a workplace setting might be different from the keigo used with a customer or client.

It's also worth noting that keigo can be difficult to master, even for native Japanese speakers. There are many different honorific expressions and rules for using them correctly, so it's important to study and practice them carefully in order to use them effectively.