Why does Это become Этого in past sense?

Question

Это достаточно: It's enough

Этого было достаточно: It was enough

I'm quite confused why the second one isn't "Это было достаточно"?

Answer 1

In Russian, short (predicative) forms of adjectives only work for definite nouns.

Машина бела means "the car is white". It only makes sense when speaking of a particular car, which would have been defined earlier in the context.

You can say something like Это не скорая помощь. Скорая помощь — белая, which would mean "This is not an ambulance vehicle. An ambulance vehicle would be white". In this case, the nominal clause скорая помощь is indefinite in the second sentence, and that's why the full form of the adjective is used, even though it's being used in predicative role.

This is one of quite rare cases when Russian shows the distinction in definiteness on grammatical level, the same thing English does with the articles "a" and "the".

Now, some adjectives, like достаточно, видно and some others do work as predicates even for indefinite nouns. But in this case, they are used in impersonal constructs, being put into singular neuter, and putting what would be the subject into the genitive case.

In addition, the adjective достаточный can only modify nouns defining measures, like срок, длина, сумма etc. Достаточный объём ведра is OK, *достаточное ведро is not.

This means that you can use personal constructs like денежная сумма достаточна для покупки дома only when the subject defines a measure and is definite.

The question какая сумма достаточна для покупки дома? would work if you are selecting the right answer from the closed list. If it's an open question, you would ask какой суммы достаточно для покупки дома. It is a very fine distinction, but it is there.

That said, the phrase это достаточно is grammatical when the word это replaces a definite noun in singular neuter which would define a measure.

A (quite contrived) example for this would be: — В таблице указаны два сопротивления, какое из них достаточно для того, чтобы не сжечь схему? — (указывая на запись в таблице) Это достаточно.

In all other cases, you have to use the impersonal construct: этого достаточно.

That's quite a high bar. All chances that это достаточно is being used incorrectly instead of grammatically correct этого достаточно.

Answer 2

I'd say that all four sentences:

Это достаточно
Этого достаточно
Это было достаточно
Этого было достаточно

sound normal, although the ones with «Этого» sounds more natural for me (both for present and past). UPD: now that I think more about this, the ones with «Это» do sound rather non-natural indeed.

The version with «Это» will be more natural if «достаточно» is a modifier to another word, when you can omit «достаточно»:

Это было достаточно интересно. = This was interesting enough. (cf. Это было интересно.)

Это достаточно интересно. = This is interesting enough. (cf. Это интересно)

Answer 3

Your issue touches upon different grammatical phenomena (to put it simpler - different parts of speech).

When the word "достаточно" (enough) is used as an adverb it can take on the role of:

1. Обстоятельства (adjunct), i.e. "У нее [имеется, было] достаточно отцовских денег" - "She has (had) enough of her father's money".
2. Именной части сказуемого (predicative), i.e. "Этих денег [будет,было] достаточно" - "This money would be (was) enough".

These both Russian sentences, as contrasted to those in English, lack verbal parts of their predicates and the implied verbs are impersonal. That is why there must be the genitive declension of nouns (pronouns) notwithstanding its time reference.

When an adjective "достаточный" is used it plays predicative and in impersonal constructions with the subject in place its shortened form "достаточен" should be employed. In this case the predicative is to be inflected according to number and person of the subject. The difference for the past tense will be lying only in the need for a verb "быть". Thus it will be:

• (for neuter nouns and pronouns) "Это [условие] достаточно" - "This [condition] is sufficient" or in plural: "Эти [условия] достаточны" and in the Past: "Это [условие] было достаточно (optional - было достаточным)";
• (for feminine nouns and pronouns) "Эта [мощность] достаточна" - "This [power] is sufficient" or in plural: "Эти [мощности] достаточны" and in the Past: "Эта [мощность] была достаточна (optional - было достаточной)";
• (for masculine nouns and pronouns) "Этот [ток] достаточен" - "This [current] is sufficient" or in plural: "Эти [токи] достаточны" and in the Past: "Этот [ток] был достаточен (optional - был достаточным)".

Answer 4

These are two different sentence constructions.

1. Это (есть) достаточно is subject + adjective / interjection; "it is enough."
   
2. Этого (есть) достаточно, этот or это is the adjective in the gen. case for the subject that is respectively omitted or replaced + adj. / interj.; "enough of that / that [something]."