We use formal English in serious situations, when applying a job, essays for school and with people we don’t know well.
Informal English is used with friends, relatives and on social media mostly.
Formal English is more common when writing, informal English is more common when we speak.
formal: I have decided to refuse his offer.
informal: I’ve decided to refuse his offer. (informal: I’ve=contraction)
formal: The person whom I met in Ireland was very friendly.
informal: The person I met in Ireland was very friendly.
(informal: relative clause without the relative pronoun whom)
formal: I ate your cake this afternoon.
informal: Ate your cake this afternoon.
formal: apologize
informal: say sorry
formal: as soon as possible
informal: ASAP
Usage Of Modal Verbs
Can I offer you something new? (neutral)
May I offer you something new? (more formal)
Might I offer you something new? (very form)