The following rules are the main uses of en and dans (there are others that are more advanced).  The accompanying questions will be based on these rules.  Fortunately, only 1-2 questions on the grammar test will test these differences.  However, knowing these rules for the oral test will certainly help with a B level and up.

En expresses the following:

1. Length or point of time (e.g., en dix minutes/within 10 minutes)
2. An action in a month, season, or year (except au printemps/in the spring) (e.g. Je voyage en mars/I travel in March. Elle arrivera en hiver/She arrives in winter)
3. “In” or “to” when le, la, l’ are not needed (e.g., Elle est en classe/She is in class—note that this means she is in class not in the class)
4. “In” or “to” when discussing provinces, some states, and countries (e.g., j’habite en Saskatchewan/I live in Saskatchewan)
5. A description or characteristics of something (e.g., en bois/of wood)
6. When talking about something wearing clothing (e.g., Il a vu une femme en jupe bleue/He saw a woman in a blue skirt)
7. When transforming something (e.g., Il sera teint en bleu/It will be dyed blue)
8. When using a gerund (-ing word) (Elle l’a dit en criant/she said it yelling)
9. In some phrases, expressions or to make connections (e.g., en question/in question, en consideration/in consideration, en détail/in detail)
10. As a preposition or replacement for a preposition (e.g., when de changes to en and is placed in front of a verb such as Pour en savoir plus/In order to know more about it)

Dans expresses/is used in the following:

1. When talking about being in a location (e.g., dans la boiîte/in the box)
2. Noting a duration of time for something that will occur in the future (e.g., Ils partent dans 5 minutes or ils partiront dans 5 minutes/they are leaving or will leave in 5 minutes—note that the verb is in present or future tense)
3. When talking about something that occurred in a decade (e.g., dans les années vingt/in the 20s)
4. In a figurative situation, location, or condition (e.g., dans les affaires/in business, dans une situation/in a situation)
5. A location when on some kind of transportation(e.g., dans l’avion/in an airplane)
6. A destination location (e.g., dans la forêt/in the forest)
7. “In” or “to” when discussing some provinces or states (e.g., Vous habitez dans le Maine/you live in Maine)
8. As a preposition for certain verbs.