Learn Romanian: Eu dormeam când ea mi-a mărturisit că este însărcinată. - I was sleeping when she confessed to me that she was pregnant.

This is a major difference between English and Romanian! In English, when we report something said in the past ('she confessed'), we shift the tense back: 'I am' becomes 'she was'. Romanian does not do this backshifting. It keeps the exact tense the person originally used. Since she originally thought/said 'I am pregnant', Romanian keeps it in the present tense: că este însărcinată.

Why do we use the imperfect tense for dormeam but the perfect compus for a mărturisit?

This sentence perfectly shows how these two past tenses work together. The imperfect (dormeam, I was sleeping) sets the background scene. It is an ongoing, continuous action. The perfect compus (a mărturisit, she confessed) is the sudden, completed action that happens during that background scene.

What is the mi- attached to the verb in mi-a mărturisit?

It is the shortened form of îmi, which is the dative pronoun meaning 'to me'. Because the next word is the auxiliary verb a (has), they combine to make pronunciation smoother, forming mi-a (to me she has).

Why does însărcinată end in -ă?

Because it is an adjective modifying ea (she). Like most adjectives in Romanian, it has to agree in gender and number with the noun it describes, so it takes the feminine singular ending -ă. Fun fact: the root of the word relates to sarcină, which means a task, load, or burden!