چرا کلمه offer ب صورت فرم گذشته میشه offered و دابل r نمیگیره؟

The r is doubled in preferred because the second syllable is stressed. In offering, offer, suffering and suffered, the first syllable is stressed so the r is not doubled. 

Frequently in English the consonant at the end of a stressed syllable is doubled. Compare “Harris,” accented on the first syllable, with “harass,” accented on the last. So you have a double “t” at the end of a stressed syllable in “committee.”

BUT, there is variance between British English and American English, so for example, in British English they spell it Travelled or cancelled but you can see canceled and traveled a lot in the US.

The stress pattern.            offered vs. preferred

Doubling final consonants before adding suffixes follows certain rules (with exceptions of course).

1)  The added suffix begins with a vowel.

2) The syllable before the suffix ends  in a single consonant preceded  by a single vowel.

3) The syllable before the added suffix is stressed.

When the verb ends in consonant + vowel + consonant, you have to double the last consonant and then add "ed" to make the Past Simple. If you have a verb with more than one syllable, you only double the consonant if the last syllable is stressed.

اگر اشتباه نکنم هر دو دو کلمه ی offer و preffer ، دو سیلابی هستن. در اولی استرس رو سیلاب اول و در اولی روی سیلاب دومه. پس r  در اولی دابل نمیشه ولی در دومی میشه.

برای آشنایی بیشتر به سیلاب ها در کلمات انگلیسی به این سایت مراجعه کنین.

https://www.howmanysyllables.com/wordprocessor