Questions & Answers about Myśleliśmy, że to prawda.
Why is it myśleliśmy with an e, when "I thought" is myślałem with an a?
This is the e/a alternation you are currently learning! The base verb is myśleć. In the past tense, the e turns into an a for most forms (like myślałem, myślała, myślały). However, the vowel switches back to e for masculine personal plural forms (groups with at least one man). These are the forms that use the letter l rather than ł, so we get myśleliśmy.
How would an all-female group say "We thought that..."?
They would say Myślałyśmy, że.... Because they are an all-female group, they use the non-masculine plural ending (-łyśmy). Since there is no l in this ending (it uses ł instead), the stem keeps the a vowel. The e only comes back before the l in masculine personal forms (myśleli, myśleliśmy, myśleliście).
The English translation says "it was true", but I don't see a past tense verb for "was" here. Why?
English has a rule where if the main verb is in the past ("thought"), the following verb shifts to the past too ("was"). Polish doesn't do this! You simply use the tense that matched your thoughts at the time. At that moment, you thought "it is true", so in Polish you just say że to prawda (that it is true), omitting the verb "to be" as is normal in the present tense.