grammar - Dependent clause tense inheritance

Is it right to tell

1. Five years ago he already knew that two plus two equals four.

or

2. Five years ago he already knew that two plus two equaled four.

?

On English classes (I live in not English speaking country) we have learnt that if present clause is dependent to past one than it turns into past. However, I did not fully agree with it and gave (the top sentence above as) an example. It caused a lot of confusion.

In my country's official pre-university English exams (one of the previous years' variants) there is a text with tasks. One of the sentences is:

Nurse believed that fresh air and food ____ very important in fighting soldiers' diseases.
A. are
B. were
...

I was really surprised when I discovered that the right answer is B. were.

My logic is: when nurse dies, it won't become unimportant. That's why A. are should be right.

Could someone please help figuring it out?

Answer

If the condition expressed in the that-clause is still true, especially if it is a "general truth", it is okay to retain the present tense.

Five years ago he already knew that two plus two equals four

As of January 17, 2015, two plus two equals four, so I would see nothing wrong with this sentence.

Let me quote from Capital Community College Foundation's The Guide to Grammar and Writing's page on Sequence of Tenses:

As long as the main clause's verb is in neither the past nor the past perfect tense, the verb of the subordinate clause can be in any tense that conveys meaning accurately. When the main clause verb is in the past or past perfect, however, the verb in the subordinate clause must be in the past or past perfect. The exception to this rule is when the subordinate clause expresses what is commonly known as a general truth:

1. In the 1950s, English teachers still believed that a background in Latin is essential for an understanding of English.


2. Columbus somehow knew that the world is round.


3. Slaveowners widely understood that literacy among oppressed people is a dangerous thing.

Thinking that a background in Latin is essential for an understanding of English was considered a general truth in the 1950, although it is not considered as such in these days.