This directly conflicts with your earlier statement that

> The infinitive form of verbs in English always starts with "to", as in "to add", "to fix" etc.

That earlier statement is wrong, so contradicting it doesn't really present a problem, but you've left me pretty confused as to what you think you're saying.

Whether "be" is the "main verb" in that sentence is a more interesting question. There's a decent argument that it is, in that "this can be a hassle" is easily viewed as a modified form of the sentence "this is a hassle", but not as a modified form of the defective sentence "this can". But you're correct that the standard analysis just says that "this can be a hassle" is sentence where the subject is "this", the verb is "can", and the object is an infinitive clause.