understand

The verb "understand" follows the ABB pattern: the past simple and past participle share one form ("understood"), while the base form ("understand") is different. ABB is the most common pattern among English irregular verbs — roughly half of all irregulars fall here, so mastering this group pays dividends across the whole language.

Use the past simple "understood" to talk about a completed action or event at a definite moment in the past. It stands alone as the main verb — no auxiliary is needed. Common accompanying time expressions include yesterday, last week, ago, and when: "She understood the whole report by herself." The action is seen as finished and anchored to a specific past time.

The past participle "understood" always works together with an auxiliary verb. It appears in: (1) the present perfect with have/has — "I have understood this several times before"; (2) the past perfect with had — "By the time we called, they had already understood"; (3) passive voice — "The decision was understood by the board." Never use "understood" as a standalone past-tense verb without an auxiliary.

Memory tip: because "understood" and "understood" are the same, you only need to memorise one past form. Many ABB verbs follow recognisable sound shifts — a large group ends in -ought (bring→brought, buy→bought, catch→caught, fight→fought, seek→sought, teach→taught, think→thought), and another group shows a vowel shift plus -t (feel→felt, keep→kept, leave→left, mean→meant, sleep→slept). Identifying which sub-group "understand" belongs to can make the form easier to remember long-term.