When to use a word for “be”
Choose เป็น, อยู่, คือ or มี, and recognise descriptions that need none of them.
English uses be before an adjective: the food is spicy, or I am hungry. Thai normally puts the description straight after the subject. อาหารเผ็ด means the food is spicy. There is no missing verb to fill in.
This food is spicy.
I'm hungry.
When you do need a verb, choose it for the relationship you want to express. A role, a location, an identification and possession don't all use the same Thai word.
Different relationships
I'm a teacher.
They're at home.
I have money.
มี expresses possession here. The subject has the money.
Where is the toilet?
Common mistake
Don't put เป็น before an ordinary description such as hungry. ฉันหิว means I'm hungry. ฉันเป็นหิว doesn't follow the Thai pattern. Learn the whole relationship: a teacher uses เป็น, hungry doesn't, and a location uses อยู่.
Practise
Kai says
ปลา means fish; น้ำปลา is fish sauce. Once you recognise the parts, a longer Thai word can start looking much less unfamiliar.