Learn Spanish (Latin America): Como tienes el día libre, es una buena alternativa. - Since you have the day off, it is a good alternative.

When como is placed at the very beginning of a sentence, it usually acts as a causal connector meaning "since" or "because". It sets up the reason for what happens in the second half of the sentence.

Could I use porque instead of como at the beginning of this sentence?

Not naturally, no. In Spanish, we generally don't start a sentence with porque to explain a reason. If the reason comes first, you use como (Como tienes el día libre...). If you prefer to use porque, you would flip the order: Es una buena alternativa porque tienes el día libre.

Why is there no word for "it" before es una buena alternativa?

Spanish frequently drops subject pronouns because the verb form already tells you who or what the subject is. The verb es already means "it is" in this context, so no extra word for "it" is needed.

Why does buena come before alternativa? I thought adjectives go after the noun, like in el día libre.

You are right that descriptive adjectives usually go after the noun in Spanish. However, common adjectives that express a subjective opinion or an inherent quality, like buena (good) or mala (bad), are very frequently placed right before the noun to sound more natural.