Question

In which case would the use of informal syntax be more appropriate than a formal syntax?

Answer 1

A Birthday card to your friend

Step-by-step explanation:

From reading 4 different answers, to break it down. Formal is using your Manners. informal is being comfortable and talking to your friend using slang

Answer 2

That would actually depend on the context and the situation. Formal syntax is obviously appropriate for formal situations, such as meetings, lectures, memorials, etc. (in spoken form), or research papers, analyses, non-fiction (in written form).
On the other hand, informal syntax would be appropriate for informal situations, such as hanging out with friends and family (in spoken form), or fiction, dialogues, etc. (in written form).