離陸の際は、シートベルトを締めなくてはならない。

takeoff

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

on the occasion of; circumstances; juncture

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

seat belt; seatbelt

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to tie; to fasten; to total; to sum; to be strict with; to economize; to economise; to cut down on; to salt; to marinate; to pickle; to make sushi adding a mixture of vinegar and salt; to wear (necktie); to put on (necktie)

if ... then; when; and; why don't you...?; the more (one does something); because; since