The word “هم” has four different important usages.
1-
Most important of all, “هم” works as a reciprocal pronoun, along with “همدیگر”. The words “همدیگر” and “هم” can be used interchangeably in this context.
ما با هم دوست هستیم.
دستِ هم را گرفتند.
چیزهای خوب را از همدیگر یاد بگیرید.
من و خواهرم همیشه با همدیگر دعوا میکردیم.
آنها بدونِ هم هیچجا نمیروند.
We are friends together.
They took each other’s hand[s].
Learn good things from each other.
My sister and I (lit. I and my sister) always fought with each other.
They don't go anywhere without each other.
2-
The word “هم” can be used to emphasize on the existence of more than one item by appearing before every item in the list.
هم حسین هم برادرش آمدند.
هم چایی میخورم هم قهوه.
امیر هم تکلیفش را نوشت، هم غذا خورد، هم خوابید.
Both Hossein and his brother came.
I’ll drink (lit. eat) both tea and coffee.
Amir did (lit. wrote) his homework, ate his food and slept. (Emphasizing that he did all the three things.)
3-
The word “هم” can simply mean “too”.
سوسن هم مریض شد.
لاکپشتها هم پرواز میکنند!
سالاد هم میخوری؟
Susan became ill too.
Turtles fly too!
Will you have (eat) salad too?
4-
“هم” can come to mean “even”, usually putting stress on the noun phrase that precedes it.
خانوادهاش هم نمیدانند او کجاست.
ویندوز را هم دوباره نصب کردم، امّا ویروس پاک نشد.
این زندگی از مردن هم بدتر است.
Even his/her family don’t know where he/she is.
I even reinstalled Windows, but the virus wasn’t removed.