I am wondering if there are any rules that determine when to use “que” or “do que” when comparing things. This is puzzling me. I am just going through adjectives in Lesson 4, I think, and I am having trouble determining when to use which. When is it that one has to use “do que” rather than just “que”?
Just a facetious example: “Uma mosca é mais pequena que um elefante!” or “Uma mosca é mais pequena do que um elefante!” Some of the examples in Lesson 4 Adjectives seem like either one would be correct, and yet only one form is shown as correct. 
Both are correct! “Do que” only becomes more recommended than the alternative (without necessarily being the only acceptable option) if the second part of the comparison includes a verb.
- Tu falas mais do que ouves. (You talk more than you listen)
- Ele é mais inteligente do que pensa. (He’s more intelligent than he thinks)
Otherwise, anything goes:
- Ela é mais alta que/do que a irmã. (She’s taller than her sister)
- Eu sou melhor que/do que ele. (I’m better than him)
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