Can you use simple tense with preposition "by"?

Question

My teacher always tells me that whenever I am using the preposition "by", I should use the perfect tense, whilst the simple tense is appropriated when I am using "in". For example:

1. By 2010, exports had shrunk to £500mln
2. Exports will reach £2bn in 2035

I am doing a writing and I am not sure if this sentence is correct:

By 1970 it [the services inductry sector] already employed more people than the other three industries combined, and in 2010 it reached its highest figure

I am using by and then a simple tense. However the following sentence doesn't sound right to me:

By 1970 it [the services inductry sector] had already employed more people than the other three industries combined

mainly because this is not true, since in previous years other industries employed a myriad of people. What I am trying to say is that in 1970, manufacturing employed more people than the other three sectors combined in 1970, not that it had employed more people taking into account the employees of the past because that is not true.

Answer 1

It isn't the tense that determines the preposition. When you say something happened in a year it means an event took place during that precise calendar year; when you say something happened by a year the inference is that there was some kind of process over time that culminated or was measured that year.

The same is true when speaking about the future. "I want to be married in 2025" would mean that 2025 is the precise you wish to get married, while "I want to be married by 2025" would mean that is the latest you wish to be married, but it could happen before.

With your specific examples:

By 2010, exports had shrunk to £500mln

This suggests exports had been shrinking for some time, but the £500mln mark was reached in 2010. If you wanted to suggest that they suddenly dropped during that year, you would say:

In 2010, exports shrank to £500mln

Your second example:

Exports will reach £2bn in 2035

This means that the £2bn mark will be reached during the year 2035. But if you wanted to say that exports are already rising and that the latest they will reach that amount is 2035, then you could say:

Exports will reach £2bn by 2035.

Answer 2

We use in to refer to longer period of time (months, years, decades, etc.)- in June, in 2035

I was born in 1975. (NOT by 1975)

He will come in 2025. (a specific year-2025, NOT 2023 or 2024)

We use by meaning 'not later than'. By refers to deadlines.

'by 2035' means in 2035 or before 2035, not later than 2035

He will come by 2025. (deadline-2025, He will come in 2025 or before 2025.)

1. By 2010, exports shrank to £500mln. (Simple past tense is appropriate.)

By 1970 it [the services industry sector] already employed...

In British English the perfect tense is used with already. But in American English simple past tense can be used with already.