Which grammar mistakes matter most in speech?

Articles and prepositions, because they appear several times per sentence and never block meaning, so nobody corrects you. Tense errors are more noticeable to a teacher but less frequent in ordinary conversation. If you only fix two things this year, fix a versus the and the preposition that follows your five most-used verbs.

They occur several times per sentence and never block understanding, so nobody corrects you and the habit survives for years. Tense errors are rarer and more often corrected, which is why they feel like the bigger problem when they are not.

Speaking English Without Grammar Mistakes: Articles, Prepositions and the Two-Second Check →