Because إلكترونيا is describing بريدا, and adjectives in Arabic agree with the noun they describe in:
So in fully vowelled form you’d see بريداً إلكترونياً.
Both are possible, but they differ in style/meaning:
إلى is a preposition meaning to, and it makes the following noun genitive.
صديق = friend.
صديقي = my friend, where ـي is the attached possessive pronoun my. When ـي attaches, it also “locks in” a vowel pattern, so you get صديقي (not صديقِي written separately).
Yes, sometimes. The nuance is:
Both are correct but slightly different:
الـ is the Arabic definite article = the.
So مكتب = an office / office, while المكتب = the office. After a preposition like من, it’s still the office, just in the genitive position.
In careful MSA with full vowel marks, you’d typically write: