Why is "happened" spelled with a single "n"? [duplicate]

I've seen the words

• plan -> planned, planning
• begin -> beginning

Why is "happen" different and why is "happenning" an incorrect spelling?

The difference between planned, penned, beginning which have double n's, and happening with a single n is the stressed syllable.

When the syllable that ends in n is stressed, we double the n if another syllable starting with a vowel sound follows it. If we didn't double it, we might feel like elongating the vowel sound of the syllable ending with n.

When the syllable is not stressed, the vowel sound is usually reduced to a schwa, and we don't seem to have the tendency to try and elongate that sound.

Other examples of an unstressed syllable ending with an n would be ripen -> ripening or open -> opening.