Obligations or the word "должен"

Hello, dear student! There is a range of words in Russian which express obligations for doing any activity. All of them are differed in meaning and connotation: in some cases you must do something and it is obligatory, in other cases words indicate a piece of advice or optional fulfillment. Let's look at the most important words which we commonly use in our speech:

"Должно" (neuter gender), "должен" (masculine gender) and "должна" (feminine gender). There is also the form "должны" for plural nouns:

Мы должны приехать туда в пять [My dalzhny priekhat' tuda f pyat'] - We must be there at 5 o'clock.

Вера должна сварить обед [Vera dalzhna sfarit' abed] - Vera has to cook lunch

Это должно было случиться [Ekhta dalzhno byla sluchit'sya] - This had to happen

These words are changed according to time when we use them (we know, that there is past, present and future tense in Russian).

The structure of present simple will be as follows:

A noun in the nominative case + должен (должна, должно, должны) (must/have to) + verb (infinitive). Please, remember that the word "должен" always agrees with a noun in nominative in gender and number:

Алёша должен закончить эту работу сегодня [Alyosha dolzhen zakonchit' ekhtu rabotu sivodnya] - Alyosha needs to complete this work today

Вы должны говорить по-русски, если хотите работать тут [Vy dalzhny gavarit' pa ruski, esli khatite rabotat' tut] - You must speak Russian if you want to work here.

As for the past tense, it should be noted that we add one more word to the structure: a noun in the nominative case + должен (должна, должно, должны) (must/have to) + быть (to be) (in the past tense) + verb (infinitive):

Лена должна была приехать [Lena dalzhna byla priekhat'] - Lena should have come

Мы должны были поехать туда [My dalzhny byli paekhat' tuda] - We had to go there

And, finally, future tense: a noun in the nominative case + должен (должна, должно, должны) (must/have to) + быть (to be in the future tense) + verb (infinitive). In the future tense, the word "быть" agrees with a noun in nominative in persons and numbers:

Соня должна будет встретить их [Sonya dalzhna budet fstretit' ikh] - Sonya should meet them

Должно быть, они купят цветы [Dalzhno byt' ani kupyat tsvety] - Obviously they will buy flowers.