Two patterns: (1)
但凡 + condition,
都/
就… — 'every X without exception':
但凡看过的人都说好 ('everyone who has seen it says it's good'). (2) Counterfactual:
但凡 X,
就 Y — 'if only X had happened, then Y'. Slightly literary; everyday speech swaps in
只要 (zhǐyào) or
凡是 (fánshì).