Used toHow to use the expression "used to" correctly
The expression used to has two meanings. When it refers to past habitual actions, the past tense used is followed by an infinitive (verb form with to):
We used to go fishing there. = In the past we went fishing there habitually.
The second meaning, in which used is a past participle and to is a preposition, is a synonym for accustomed to:
The people in that desert region are used to the extreme heat. = the people are accustomed to the heat
In both cases, used always takes the past tense/past participle suffix:
use to go fishing
used to go fishing
use to the heat
used to the heat
This confusion is understandable in that, like the expression supposed to, the “d” at the end of used and the “t” sound at the beginning of to tend to blend together, so after hearing it pronounced that way many times, it’s easy to forget that the -ed suffix is there.
Common Errors in English Usage: Errors in diction and idiom commonly made by native speakers of English
List of Common Errors in English Usage (PDF): Printable version of the complete list
Common Grammar Errors: A list of common errors in grammar (topics like subject-verb agreement and parallelism) as distinct from usage