Question

the rule
placing a comma after an introductory clause lets the reader know that the introduction has come to an end and the main clause is about to begin.
example
after the trip, we needed to unpack our luggage.
or
we needed to unpack our luggage after the trip.
ask yourself...
can the part of the sentence before the comma move to the end of the main clause and still make sense? if the answer is yes, you are dealing with an introductory clause.
practice
add commas to the following sentences if they are needed. not every sentence will need commas.

1. because the game was so one - sided we left early.
2. wading into the cool lake we found relief from the heat.
3. the alarm clock no longer worked after the power outage.
4. according to my dad our local football team is the best in the country.
5. if it is raining we will need to move the wedding indoors.
6. even if you are afraid of flying you should embrace the opportunity to travel.
7. after the championship game the team went out for ice - cream.
8. before the school year started i had to complete my summer homework.
9. we adopted a pet dog after moving out of the city.
10. when visiting a new country make sure you are familiar with the local customs.

Explanation:

Each sentence is checked by moving the potential introductory part to the end to see if the main clause still makes sense. If yes, a comma is added after the introductory clause/phrase.

Answer:

1. Because the game was so one-sided, we left early.
2. Wading into the cool lake, we found relief from the heat.
3. The alarm clock no longer worked after the power outage. (No comma needed)
4. According to my dad, our local football team is the best in the country.
5. If it is raining, we will need to move the wedding indoors.
6. Even if you are afraid of flying, you should embrace the opportunity to travel.
7. After the championship game, the team went out for ice-cream.
8. Before the school year started, I had to complete my summer homework.
9. We adopted a pet dog after moving out of the city. (No comma needed)
10. When visiting a new country, make sure you are familiar with the local customs.