Learn Polish: Ich małżeństwo było bardzo szczęśliwe.

The suffix -stwo is used to create abstract or collective nouns. Here, it is attached to the root of małżonek (spouse) to create the abstract concept of the relationship: marriage. Because it ends in -stwo, the noun is strictly neuter.

Does małżeństwo only mean the abstract concept of marriage?

Not necessarily. The -stwo suffix is also used to form collective nouns representing groups of people. Because of this, małżeństwo can also mean a married couple. In this sentence, it likely refers to the marriage itself, but saying the married couple was very happy also makes perfect sense.

Why are we using the neuter adjective szczęśliwe and the neuter verb było?

They must agree grammatically with małżeństwo. Even though a marriage involves living people, the word itself is grammatically neuter because of the -stwo ending. Therefore, it requires the neuter past tense verb było and the neuter nominative adjective szczęśliwe.

Why doesn't the word ich (their) have a neuter ending to match małżeństwo?