1. Forms of the Infinitive

The Infinitive being one of the basic verb forms has the categories of aspect and voice and hence has the following forms:

1. the simple (indefinite) infinitive: (to) obtain

2. the progressive (continuous) infinitive: (to) be writing

3. the perfect infinitive: (to) have obtained/written

4. the perfect progressive infinitive: (to) have been writing

5. the simple (indefinite) infinitive passive: (to) be obtained/written

6. the perfect infinitive passive: (to) have been writing

The simple infinitive and the simple infinitive passive express an action simultaneous with the action expressed by the finite verb. It can refer to the present, past or future:

He is sure to know that.

He was too tired to solve this problem.

He will be happy to help you.

The progressive infinitive shows that the action is in progress; it is viewed as incomplete at a certain time-point:

He seems to be solving the task right now.

Note that there are some verbs which are not normally used in the progressive form; they do not have the progressive infinitive form either. Instead, the simple infinitive is used:

He appeared to be watching us.

He appeared to see what we were doing.

The perfect infinitive and the perfect infinitive passive describe an action that precedes the action expressed by the finite verb:

He was happy to speak to us.

He was happy to have spoken to us.

The perfect infinitive is widely used with modal verbs to denote unrealized actions in the past (I could have solved it.) or to express criticism (It should have been done long ago.)

The perfect progressive infinitive indicates that an action in progress began some time before the action expressed by the finite verb:

He admitted to have been preparing this experiment for a long time.

The above-mentioned forms are called the full infinitive as they are used with to. The infinitive without to is called the bare infinitive and is usually used after modal verbs; sometimes after the verb need; after let and make followed by an object: Let him speak. They made me do it.; after the verbs feel, hear, see, watch, etc. in the infinitive construction Complex Object: I heard them talk about it.; after modal expressions had better and would rather, as well as rather than: Rather than waste time doing it manually, let’s use the computer$ and in some other situations.

2. Functions of the Infinitive in the sentence

The Infinitive can be used in sentences in different syntactic functions:

1. as the Subject: To speak to him was something they didn’t risk doing.

It would be a pity to miss such an opportunity.

2. as the Predicative (commonly after the link verb be):

The difficulty was to find all the solutions quickly.

3. as part of the compound modal verbal predicate:

This equation can be obtained by differentiating…

4. as an Object used either directly after verbs, or after a verb + object, or after predicative adjectives, or preceded by a wh-word:

We wanted to see it. He refused to be assisted.

We asked them to help us.

I am curious to know the result. I was happy to have been invited to that conference.

I cannot decide which approach to use in this case.

5. as an Attribute after nouns or pronouns:

They have set up a program to solve such equations.

He had something to say.

They spoke about the conference to be held in September.

6. as an Adverbial Modifier of purpose, result, etc.

He went to the conference to present the results of his work.

She switched off the telephone in order not to be disturbed.

7. as Parentheses (a set phrase placed usually at the beginning of a sentence):

To start with, the explanation was obvious.

To be more exact, it cannot be solved in this way.

Exercise 1. Identify the forms and the functions of the Infinitive in the following sentences:

1. It is extremely useful to recognize alternative forms of the general solution of the differential equation…

2. It is easy to see what the graphs of such functions look like.

3. The solution can also be written in the form of equation (2).

4. To put this equation in the form of (1), we set …

5. We need only set c = 0 in (1) and (2).

6. To see that this is so, we expand (1) using the trigonometric identity for the sine of the sum of two numbers.

7. Consider an equation F (x,y,z)=0 in which not all the variables need occur.

8. The general problem, given a differential equation, is to find all its solutions.

Exercise 2. Translate the following sentences into English using the Infinitive (in different forms and functions).

1. Легко вывести необходимое условие для того, чтобы эта форма была полным дифференциалом.

2. Для того чтобы удовлетворялось уравнение (2), необходимо принять, что…

3. Как это можно доказать?

4. Что следует сделать, если мы хотим показать, что дифференциал не является полным?

5. Это выражение следует записать в стандартной форме.

6. Более высокие частные производные можно получить путем дифференцирования частных производных первого порядка. (*the first order partial derivatives).

7. Равенство, которое следует получить, должно удовлетворять следующим условиям…