Retrieve the language
Past continuous (was/were + -ing) sets an action in progress; past simple reports a completed event: While we were packing, the organiser called. When does not automatically require one tense: meaning decides. An -ed adjective describes a feeling (interested), while -ing describes its cause (interesting). Useful is helpful; useless is not effective, not morally worthless.
Present perfect: have/has + past participle connects past experience or an unfinished period to now: We have tested the device twice. Use past simple with a finished past time: We tested it yesterday. A gerund names an activity after enjoy or a preposition: useful for recording. A to-infinitive can express purpose: We use it to record instructions.