Why can't I say "He felled off a ladder"? I know I should use fell, but it has happend in the past, so I will use felled because that's the past tense of fell.

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English has regular verbs and irregular verbs. A "regular" verb means that it follows the usual pattern. An "irregular" verb means that it doesn't follow the usual pattern. "Fell" is an irregular verb.

An example of a regular verb is close, as in "I closed the door." Most verbs are regular and the follow the same pattern as close.
present tense: I close, he closes
past tense: closed

But fell is irregular.
present tense: I fall, he falls
past tense: fell (not "falled" or "felled")