unterstrich

In a sentence

• Der Benutzername enthält einen Unterstrich.

  The username contains an underscore.

  Here, 'unterstrich' is used as a noun, meaning 'underscore', and refers to a character in a computing context.

• Ich habe den wichtigen Punkt im Text unterstrichen.

  I underlined the important point in the text.

  Here, 'unterstrich' is the past tense of 'unterstreichen', meaning 'to underline', and describes marking something in text.

• Der Lehrer unterstrich die Bedeutung von regelmäßigem Üben.

  The teacher emphasized the importance of regular practice.

  In this instance, 'unterstrich' is the past tense of 'unterstreichen', used metaphorically to mean 'to emphasize' a concept.