Subjective meaning of modal verbs

Modal verbs, when used with their subjective meaning, refer mostly to the present time.

Examples	time
Paul muss zu Hause sein.	present
Paul soll zu Hause sein.	present

When, however, the hypothesis or the reported statement of someone refer to the past, the modal verb is combined with a perfect infinitive (Infinitiv II).

Examples	present	past
Paul muss zu Hause	sein	gewesen sein
Paul soll zu Hause	sein	gewesen sein

If the hypothesis or the reported statement are made in the past, the modal verb is in the Präteritum and is connected either with a present infinitive (simultaneity) or a perfect infinitive (anteriority). Under no circumstances is the modal verb in the present perfect, past perfect or in the future tenses.