Benefitted vs. Benefited

The basic rule when building the past-tense form of a regular verb is to add "-ed". A special rule, anyway, is used when we're talking about a monosyllabic verb that ends in this pattern: consonant-vowel-consonant. In this specific case, doubling the last consonant is required before adding the "-ed", to correctly spell the past-tense form of the word.

So why aren't things clear when applying these simple rules for the verb "benefit"? Which past tense is the correct one, "benefitted" or "benefited"? You will find out right now!

Benefitted vs. Benefited

Both forms are accepted, you have probably observed that already, because you saw this word written both with a single and a double "t". Here's why: "benefit" is a regular verb with 3 syllables, so the immediate result when creating the past-tense form is obtained by simply adding the "-ed". This is how you get the "benefited" version, which is correct.

But the word benefit also ends in the syllable "fit". Another verb itself, "fit" is regular and monosyllabic, respecting the pattern consonant-vowel-consonant. Therefore, it doubles its last letter before adding the "-ed" suffix. That's how its past tense is "fitted" and also why people assume that "benefit" will follow the same rule, turning into "benefitted" at the past-tense. This form is also correct.

When do we use "benefitted"?

You can use it anytime, in any context, as the meaning of the word remains the same regardless of how many "t"-s are used. Double "t", anyway, is more commonly preferred in British English.

When do we use "benefited"?

Again, it is up to you when and where you want to spell the verb with a single "t". But if you choose this version, you will more likely be part of the majority of American English speakers, who tend to avoid double letters whenever possible, in order to make the spelling (both written and spoken) easier.

Conclusion

So if you will ever ask yourself again if you should spell "benefitted" or "benefited", the answer is "benefitted" AND "benefited". Both are correct and it is up to your personal preference which you'll use.