Per comments, OP's instructors are probably just incompetent (or perhaps he somehow misunderstood what they meant). In standard English, ***and*** has two distinct meanings... > 1: *Dick liked Jane **and** she liked him* 2: *Dick insulted Jane **and** she slapped him* ...where #1 simply uses ***and*** to join two "independent" statements - stripped of pronoun substitutions,... > 1a: *Dick liked Jane* ***and*** (another statement, with no specific relationship to the first) 1b: *Jane liked Dick* But in #2 the conjunction implies a temporal/causal relationship between the two statements... > 2a: *Dick insulted Jane* ***and*** (*subsequently* - pragmatically, probably *consequently*) 2b: *Jane slapped Dick* Grammatically/semantically, the ***and*** in #2 can be replaced by ***and then*** or simply ***then***. Perhaps OP's instructors felt he was overusing the two-word form in contexts where it was pragmatically obvious that the second event was *later in time and/or caused by the preceding event*. But this is a matter of style, nothing to do with grammaticality. --- TL;DR: Even if OP's instructors knew what they were talking about, they've obviously failed to convey the correct information to their student. The only reasons to avoid ***and then*** are (1) - if the context implies no [con]sequential relationship, and (2) - because using it *excessively* can result in a stilted prose style.