When seen in sentences containing verbs negated with the 助動詞（じょどうし） (auxiliary verb) ない, とても carries the nuance of ‘not (A) at all’, or ‘cannot (A) at all’. とても is supposed to only be used when expressing 不可能（ふかのう） ‘impossibility’. Due to this, it will primarily appear alongside the potential form of verbs.
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その考（かんが）え方（かた）はとても理解（りかい）できない。
I cannot understand that way of thinking at all.
彼（かれ）はいつも嘘（うそ）ばかり言（い）うから、彼（かれ）の言（い）う事（こと）はとても信（しん）じられない。
He always just lies, so I cannot trust what he says at all.
As long as some part of the sentence expresses the speaker's view about the impossibility of (A), the verb is not always required to be in the negative form.
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彼（かれ）に子供（こども）ができたの！？彼（かれ）が親（おや）になることを想像（そうぞう）するのはとても無理（むり）だ。
He has a baby coming? I cannot imagine him becoming a parent at all.
どこの航空会社（こうくうがいしゃ）も今（いま）はチケットを売（う）っていないから、旅行（りょこう）をするのがとても無理（むり）だ。
No airline company is selling airline tickets, so it is not possible to travel at all.