تفاوت بین دو واژه ی formal و official

Formal typically refers to something that is:

Polite, courteous, and respectful
Serious, dignified, and solemn
Following established rules, customs, or protocols
Not informal or casual

The formal invitation to the wedding reception was sent out a month in advance. (Here, "formal" refers to the polite and respectful nature of the invitation.)

• The formaldehyde in the museum preserved the ancient artifacts for centuries. (Here, 

  "formaldehyde" refers to a chemical substance used in a formal or serious context.)

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Official, on the other hand, typically refers to something that is:

Authoritative, sanctioned, or approved by an organization or government

Legally binding or recognized

Carrying weight or significance due to its connection to an authority figure or institution

The official language of the country is English. (Here, "official" refers to the language that is recognized and authorized by the government.)

The official report from the investigation revealed new evidence. (Here, "official" refers to the report that is sanctioned and approved by the investigation team or authorities.)