Learn Spanish (Latin America): No puedo enviar el mensaje, pero lo guardo. - I cannot send the message, but I save it.

lo means "it" in this sentence. It replaces el mensaje (the message). Because el mensaje is a masculine noun, we must use the masculine pronoun lo. If we were talking about a feminine noun like la carta (the letter), we would use la.

In English we say "I save it", but here it says lo guardo ("it I save"). Does the pronoun always go before the verb?

Yes, when you have a single conjugated verb like guardo (I save), the direct object pronoun like lo must go right before it. Unlike in English, you cannot put it after the verb here.

Why is it puedo enviar instead of puedo envío?

When two verbs go together to form one idea like "I can send", only the first verb changes its ending to match the person doing the action. puedo is the "I" form, and it is followed by the unchanged dictionary form (the infinitive) of the second verb, enviar (to send).

The word mensaje ends in an "e". How do I know if it is masculine or feminine?

You can tell it is masculine because it uses the masculine word for "the", which is el. As a handy tip, almost all Spanish nouns that end in -aje are masculine.