势必, 必然, 未免 — bound to, and rather too

The rule 势必 = is bound to — about a FUTURE CONSEQUENCE: 这样下去势必出问题。
必然 = inevitable — an objective law, and it can be an adjective: 这是必然的结果。
难免 = hard to avoid — something unwelcome but forgivable: 第一次做，难免出错。
未免 = rather too — a SOFT COMPLAINT, not unavoidable: 这样说未免太过分了。
未免 nearly always takes 太 or 有点儿 after it. Do not confuse it with 难免; they are opposites in tone.