Learn German: Zweitens wollte er unser Potenzial zeigen. - Secondly, he wanted to show our potential.

The word zweitens is an adverb meaning 'secondly', created by adding the suffix -ns to the ordinal number zweite. Because it occupies the first position in the sentence, the conjugated verb wollte must come immediately after it to satisfy the strict verb-second rule. This pushes the subject er into the third position.

Why does the possessive determiner unser not have an ending here?

The word unser modifies the noun Potenzial, which is neuter (das Potenzial) and serves as the direct object of the sentence, putting it in the accusative case. In German, neuter nouns in both the nominative and accusative cases take the base form of possessive determiners, so unser gets no extra ending here.

Is the spelling Potenzial correct, or shouldn't it be Potential like in English?

Both spellings are actually correct in German. However, Potenzial with a z is the recommended and most common spelling today. It was updated to reflect the German pronunciation, where the 't' before an 'i' in loanwords makes a 'ts' sound, which is exactly how a z is pronounced in German.

Why is the simple past form wollte used here instead of the spoken past?