Collocations for underline - verb

heavily
The word ‘not’ was heavily underlined.

clearly
just, merely, only, simply
This disaster merely underlines the need for caution.

further (BrE)

serve to
seem to

underline the fact that …

Underline is used with these nouns as the object:
argument, credentials, difference, extent, fact, folly, importance, need, point, relevance, superiority, value