The tense that we should use to give a reason for a past event

Question

Suppose that we have run a process in the past, but we are able to re-run the process whenever we want in the future and always the same results would emerge. If we want to provide reasons for the occurrence of these results, what tense should we use? Should we use past tense since the process has run in the past? Should we use present simple and give a reason as a general statement since we can reproduce the results?

As the process proceeded, these values became lower. The reason for this is/was that they are/were highly dependent on the resources which are/were consumed as the process proceeds/proceeded.

Answer 1

Both forms are possible as long as you are consistent.

As the process proceeds, these values FALL. The reason for this IS that they ARE ...etc.

or

As the process proceeded, these values FELL. The reason this WAS that they WERE etc...

I prefer the first form as it describes a process that remains true in all cases.

The second takes the form of a report on an experiment rather than a statement of a consistent result.

Both are legitimate.

Answer 2

It depends on the reason you're giving the reasons, and it's no different from the normal meanings of the various tenses.

Are you describing a past event and saying why it happened that way? Then use simple past.

Are you describing the general fact of how the event happens and what its causes are? Then use simple present.

Are you describing how things happen to someone who might witness them or might participate in them in the future and you want them to know the reasons these things happen? Then use future tenses.

Answer 3

For the sentence in your question,

As the process proceeded, these values became lower. The reason for this is/was that they are/were highly dependent on the resources which are/were consumed as the process proceeds/proceeded.

you can use consistent 'past tense' or consistent 'present tense'. e.g.

As the process proceeded, these values became lower. The reason for this is that they are highly dependent on the resources which are consumed as the process proceeds.

As the process proceeded, these values became lower. The reason for this is that they were highly dependent on the resources which were consumed as the process proceeded.

As the process proceeded, these values became lower. The reason for this was that they were highly dependent on the resources which were consumed as the process proceeded.

The reason for this is the specialness of the "be" verb. Used alone "be" states things in simple truth versions. In "There was an .." the verb "be" is talking about a state of things that was in the past, and is no longer in the present. In "..became lower..", the state of things became that state of things and it still is in the present.

This sentence

As the process proceeded, these values became lower. The reason for this is that they are highly dependent on the resources which were consumed as the process proceeded.

is also grammatically correct. You can separate the two clauses "The reason for this is that they are highly dependent on the resources" and "which were consumed as the process proceeded.". You can use "is" or "was" with reason provided the reason has "not expired"(the reason is no longer valid). If the reason is no longer valid, you have to use the past tense.