Learn Russian: Это мама.

In Russian, the present tense verbs for "to be" (am, is, are) are almost always skipped. You just put the pointing word and the thing it equals side by side. So, Это мама literally means "This mom", but it translates perfectly to "This is mom."

Since мама is a feminine word, shouldn't это change to match its gender?

Not when you are using it to say "This is..."! When это is used to introduce someone or something, it acts as a fixed pointing word. It stays exactly as это whether the noun after it is masculine, feminine, neuter, or plural.

How do we know if it means "This is mom" or "This is a mom"?

Russian doesn't have words for "a", "an", or "the" (called articles). You just use the noun by itself. Depending on the situation, Это мама could mean "This is mom", "This is a mom", or "This is the mom"—context tells you which one makes sense!

Why does the о at the end of это sound like an "a"?

In Russian, the letter о only makes a clear "oh" sound when the spoken stress (emphasis) is on it. In the word это, the stress is on the first syllable (э). When an о is unstressed, its pronunciation weakens into an "ah" sound.