The rule is, you should try to explain your new-learned words in English on your own. After this step you should compare your explanation with the explanation in English-to-English dictionary and see what's the difference

thereafter

My explanation: from then on

Dictionary: from a specified time onward; from then on

precursor:

My explanation:  if something A will easily turn into B, then A is called a precursor of B.

Dictionary: a person or thing that comes before another of the same kind; a forerunner.

repulsive

My explanation: disgusting

Dictionary: causing repugnance or aversion; disgusting

crop up

My explanation: appear suddenly

Dictionary: to appear unexpectedly or occasionally

deliberate

My explanation: do something with enough consideration

Dictionary: arising from or marked by careful consideration

aroma

dictionary: a pleasant smell that is easy to notice

my explaination: a smell that makes people feel pleasant and comfortable