Khan Academy
Khan Academy: The Vowel Shift Irregular Verb
This group of irregular verbs have vowel sounds that change in a predictable way, when they go from the present to the past tense.
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Khan Academy: The Past Tense
The past tense refers to event that have happened in the past. The basic way to form the past tense in English is to take the present tense of the word and add the suffix -ed. For example, to turn the verb 'walk' into the past tense, add...
Khan Academy
Khan Academy: The Funky Ed Irregular Verb
These are the most regular of the irregular verbs; while they don't behave exactly like regular verbs, like present tense 'walk' become past tense 'walked', they do maintain the '-ed' sound. It's just spelled differently.
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Khan Academy: Introduction to Verb Aspect
If verb tense allows you to control the past, the present, and the future, then aspect gives you even finer control over time. David, Khan Academy's resident grammarian, explains.