When you want to say that you have an experience of doing something in the past - use the word เคย (koiie) + verb.
For example: "I've been to Thailand" would be ฉันเคยไปประเทศไทย (chăn koiie bpai bprà-têt tai).
IMPORTANT: เคย (koiie) is a "past tense" maker. For example "I ate sushi yesterday" = เมื่อวานฉันกินซูชิ (mêuua waan chăn gin soo-chí) BUT "I've eaten sushi before" would be ฉันเคยกินซูชิ (chăn koiie gin soo-chí).
You can think of "I've eaten sushi before" as "I have had the experience of eating sushi in the past".
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To say you've never done something in the past, use ไม่เคย (mâi koie) + verb. For example:
I have never drank beer = ฉันไม่เคยดื่มเบียร์ (chăn mâi koie dèum bia)
เคย (koiie) + verb = ever done something | ไม่เคย (mâi koie) + verb = never done something
