cost

The verb "cost" belongs to the AAA group — all three principal parts are identical: "cost" / "cost" / "cost". The spelling never changes, whether you use it in the present tense, past simple, or as a past participle in a perfect or passive construction.

Use "cost" in the present to describe habits, facts, or scheduled events: "We cost the figures every quarter." The same form serves as the past simple for completed actions: "They cost the budget before noon." With auxiliary verbs it functions as the past participle: "She has already cost her quota." Because all three forms look the same, an auxiliary (have, has, had) or a past-time marker (yesterday, last year) provides the tense signal.

AAA verbs are mostly short, high-frequency words ending in -t or -d, such as cost, cut, hit, hurt, let, put, set, and spread. Because they never inflect, you only need to learn one spelling. The difficulty is that context — an auxiliary verb, a time adverb, or the surrounding narrative — must indicate which tense is intended.

Quick-reference rule: when "cost" follows have, has, or had, it is acting as a past participle ("she has cost the price twice"); when it follows did not or appears in a question with did, it is the infinitive in a past-tense structure ("did they cost it correctly?"); in all other uses it is most likely present simple or a base-form infinitive.