The underlines you see were added by the author to emphasize the key points.

I underlined all the words I didn't know.

Translate the underlined part.

I underlined all the words that I didn't know.

He underlined the urgent need for immediate action.

I put an underline under the word "Alice" to put an emphasis on it.

Translate the underlined sentences.

Underline all the verbs in this text which are in the future tense.

We should underline every important word.

Correct the underlined words.