Two things, and the first one falls short of the second on some quality.
序 · The move
1Put the one that holds less of the quality first — that is A.Is A really the lower one? Swapping A and B reverses who falls short.
2After 沒有, name the standard B is measured against.B is the higher mark, the line A fails to reach.
3Drop in 這麼 (near) or 那麼 (far) to point at B's degree, then close on the bare quality.No 很 before the quality, and no 比 — 沒有 already carries the comparison.
例 · Examples
1事情the matter (A)沒有is not as much as你想的what you imagined (B, the standard)那麼that (much / degree)嚴重serious (the quality)。
The matter is not as serious as you imagined.
界 · Boundary
A比B
A比B ranks A above B (A is more X); A沒有B(那麼)X ranks A below B (A is not as X). Same pair, opposite direction — the one named first wins under 比, loses under 沒有.
跟…一樣
跟…一樣 says the two match on the quality; A沒有B(那麼)X says A falls short of B. Equality against shortfall.
沒(有) before a noun denies you hold it or that it is there (沒有錢, 沒有公園). The same 沒有 between two things, closing on a quality, denies that A reaches B's level. Possession against comparison.
事情沒有你想的那麼很嚴重 → 事情沒有你想的那麼嚴重 (no 很 inside the comparison)
事情不有你想的那麼嚴重 → 事情沒有你想的那麼嚴重 (the comparison is barred with 沒有, never 不有)
事情比你想的沒有那麼嚴重 → 事情沒有你想的那麼嚴重 (use 沒有 alone for the negative; do not stack it on 比)
English 'not as serious as' keeps the quality up front and the standard last; Chinese sets B with 沒有 in the middle, points at the degree with 這/那麼, and lets the bare quality close the line.