Learn Portuguese (Portugal): Eu investiria num mercado novo.

For regular verbs like investir, you form the conditional by taking the entire infinitive word (investir) and adding the conditional endings. For eu (I) or ele/ela/você (he/she/you), the ending is -ia, making investiria (I would invest). The best part is that these endings are exactly the same for -ar, -er, and -ir verbs!

Does investiria share its ending with another tense I've learned?

Yes, this is a very common trap! The conditional endings (-ia, -ias, etc.) look exactly like the Imperfect past tense endings for -er and -ir verbs. The crucial difference is that for the Conditional, you attach them to the full infinitive (investir + -ia = investiria, meaning I would invest). For the Imperfect past, you remove the -ir first (invest + -ia = investia, meaning I used to invest).

Why do we use num here instead of just em?

In Portuguese, the preposition em (in/on) almost always merges with the indefinite article um (a/an) to form num (in a). Because mercado is a masculine singular noun, we use um, which combines with em to give us num. If it were a feminine noun, we would use numa (em + uma).