The difference between formal and informal English is not a difference between correct and incorrect, but a difference of what is known as register. A register is a variety of language related to a particular subject matter or area of activity, a set of words and expressions as well as syntactical features that may be said to characterise that specific area of language.
There are many registers: technical, academic, mathematical, scientific, etc. Very broadly speaking, we can also speak of a “formal” and “informal” register in English. In writing academic reports and the like, it would be normal to draw most of the vocabulary and expressions from the formal register, and few, if any, from the informal. This entails avoiding colloquial (everyday) or slang expressions in your writing assignments.
The question of register is far more complicated than indicated here; for example, there are many degrees of formality and informality. However, below are listed a few examples which may be of practical assistance.
Active and Passive voice
(i) Our technician repaired the fault on 12th June. Now it’s your turn to pay us.
(f) Although the fault was repaired on 12th June, payment for this intervention has still not been received.
Phrasal verbs and Latina
(i) The company laid him off because he didn't work much.
(f) His insufficient production conducted to his dismissal.
Direct and Formulaic
(i) I’m sorry but …
(i) I’m happy to say that …
(f) We regret to inform you that …
(f) We have pleasure in announcing that …
Use of Slang
(i) He had to get some money out of a hole in the wall …
(f) He withdrew the amount from an ATM.
Personal form & nominators
(i) If you lose it, then please contact us as soon as possible.
(f) Any loss of this document should be reported immediately …
(i) The bank can’t find the payment you say you’ve made.
(f) Notwithstanding that the payment has been sent the bank fails to acknowledge it.
(i) Anybody or any company.
(f) … any natural person who, and any legal entity which …
(i) If you need any help give us a call.
(f) Should you require any assistance, please feel free to contact us …
Singular & Plural Person
(i) I can help you to solve this problem. Call me!
(f) We can assist in the resolution of this matter. Contact us on our toll-free number.