Learn Portuguese (Portugal): Prefiro viajar com a família, não sozinho. - I prefer to travel with the family, not alone.

In English we often just say 'with family', but Portuguese usually requires the definite article (o, a, os, as) before nouns. So, you must say com a família, which literally translates to 'with the family'.

Since this lesson is about words like comigo, how would I say 'I prefer to travel with you'?

To say 'with you' to a friend, you would replace com a família with the special combined word contigo. The sentence becomes Prefiro viajar contigo, não sozinho. Remember, you never say com tu!

Why doesn't viajar change its ending here?

Because it immediately follows another verb (prefiro) that is already conjugated to match 'I'. When you link two verbs together like this to say 'I prefer to travel', the second verb stays in its dictionary form (the infinitive), which is viajar.

If a woman is speaking, does sozinho change?

Yes! The word for 'alone' changes its ending to match the gender of the person it describes. A man says sozinho, but a woman must say sozinha. If a group of people is speaking, they would use the plural forms sozinhos or sozinhas.