Forget the "no longer". That's completely irrelevant.
You need to know some basic grammar to figure out whether to use 'to be'.
'To be' is only ever used in the following situations:
1) To equate a noun with another noun or an adjective ex. "I am a student" (我是学生) or "I am tall" (我很高).
2) To describe a continuous tense. ex. "I am studying" (我正在学习呢)
3) To describe the passive voice. ex. "I was hit by a car" (我被汽车撞了)
It's important that you memorise these. Overusing 'to be' is a very common mistake by Chinese speakers.
If none of those apply, then you can't use 'to be'. It's as simple as that.
The continuous form of a verb is formed by adding 'ing' to the end. The general form is:
Subject + to be + verb-ing
This is more or less equivalent to 正在 before a verb in Chinese.
The passive voice generally has negative implications in Chinese, but it's completely neutral in English. "我被老师教书" sounds weird because being being taught isn't a bad thing, but "I was taught by a teacher" is perfectly natural in English. The passive voice is used as follows:
Subject + to be + perfect form of verb
If you're not using a verb in the passive voice or in a continuous tense, it's incorrect to use 'to be'. So you can never say something like "I am like" or "I am go".
Present simple is used to describe an action that happens frequently, but is not happening at the present time. To use it, you just have to put the verb after the subject.
You can add 'no longer' after you've figured out whether or not to use 'to be'.