(1) Please don't talk rubbish before you examine the issue thoroughly. (有点凶)
(2) Please don't open your mouth before examine the matter closely. (有礼貌)
问题: problem; issue; case; matter
乱说: talk rubbish=talk nonsense=胡说八道,废话;
检查清楚: examine/check thoroughly(=clearly=carefully=closely);
Please don't go shooting your mouth off about it before you take a clear check.
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Please don't talk nonsense before you've checked this problem cleary.