Means of expressing modality in Modern English

2.1 The Verb Must and Need
The verb must be the assumption, usually based on knowledge and almost bordering on certainty. The combination of the verb must non perfect with infinitive means that the alleged action (or State) is simultaneous with the assumption and the assumption can refer both to the present and past time.
Your father must be nearly eighty now.
Вашему отцу теперь, должно быть, восемьдесят лет.
You must be hungry after your long walk.
Вы, должно быть, сильно проголодались после такой долгой прогулки.
We must have taken a wrong turning.
Мы, должно быть, не там свернули.
The verb must is used in the meaning of assumptions, the probability, that a great degree of confidence. A speaker believes it is plausible hypothesis. In this sense, the verb must roughly correspond with the help of: evidently, apparently, certainly, most likely, probably. [7,84]
For the expression must have used assumptions:

1. 
   For the expression: combined with Indefinite Infinitive to express the assumptions relating to the present. Must in this case translated into Russian language should be, probably, and infinitive-a verb in the present tense. The combination of a must with infinitive can be translated into the Russian language is also a must with the infinitive:
   

He must know her address. Он, должно быть (вероятно), знает её адрес.
He must be in the Library now. Он, должно быть(вероятно), сейчас в библиотеке.
After Continuous Infinitive is a must when it is suggested that the action happens at the moment of speech:
Where is he? - He must be walking in the garden. Где он? - Он, должно быть, гуляет в саду.
Also must in the sense of probability is used with the infinitive of the verbs, the simplest form is not usually used in the form of continuing (to be, to think, etc.)

2. 
   In combination with Perfect Infinitive to express assumption that belongs to the past. The combination of the verb must with perfect infinitive means that the intended operation is preceded by a statement of assumptions, while the assumption may refer both to the present and past time. Must in such cases is being translated into Russian language through should be, probably, and infinitive-a verb in the past tense:
   

They must have forgotten to send us a copy of the telegram with their letter. Они, должно быть (вероятно), забыли приложить копию телеграммы к своему письму. The cases must have been damaged during the unloading of the vessel
Ящики, должно быть (вероятно), были повреждены во время разгрузки судна.
K.N. Kachalova specifies that for assumption expression, relating to the future, must is not used. Such Russian sentences, as
Погода, должно быть (вероятно), изменится завтра.
Лекция, должно быть (вероятно), будет интересной it is translated into English:
The weather will probably change (is likely to change) to-morrow.
The lecture will probably be interesting (is likely to be interesting). It is important to note that must verb in value of the assumption is used almost only in affirmative sentences. In negative sentences it is not used absolutely, and in interrogative sentences meets seldom, and these questions have rhetorical character. Such Russian sentences, as:
Он, должно быть, не знает об этом.
Он, должно быть, не видел её.
He probably doesn’t know about it.
He probably didn’t see her.
In negative sentences the assumption is expressed by means of the modal word evidently. [7,97]
Evidently, she did not know my address.
When the verb need has modal value, it functions in negative sentences and in combination with denial means a non-obligation of that this action (or a state) takes place in the present, will take place in the future (at not perfect infinitive) or took place in the past (at a perfect infinitive).
It needn`t take you very long. – «…это необязательно займёт много времени (это необязательно должно занять много времени)».
The verb need in value of the assumption is used quite seldom, though is met both in art, and in scientific literature (mainly in humanities). [9,58]
To compound modal predicate with the verb need with denial in value of the assumption there corresponds in Russian the simple predicate expressed by a verb in the real, future or past tense (depending on an infinitive form) to which the words "not necessarily" or a combination of the words "not necessarily has to" and an infinitive precede. [ 9,61]