I studied that とたん must be used after a verb in the past form, and that, also the clause that follows it is in the past form. In this case it bears the meaning of "as soon as something happens, something else also happens". However, I saw that とたん can also be preceded by a present tense and that the following clause can be present too, but I'm not sure if the meaning of とたん then changes. As in:「会社に入る途端に勉強しなくなる人が多い」. Could somebody provide me some examples to figure it out?
とたん（途端）is used like:
- "as soon as (I said something)"
- "just as (I said something)"
- "the instant (I said something)"
- "the moment (I said something)"
The construction is:
[verb in plain past] とたん、next sentence
The instant he said that, he stopped talking.
Sometimes とたんに is used. Then you don't use a comma.
As soon as they are done with their work, they will be killed. ;(
Wheter you use とたん or とたんに doesn't matter. The meaning wont change.
Also, to your original question: Wheter the second phrase is in the past or not won't change the meaning of とたん. It will simply only change the meaning of the sentence.
Like for example:
As soon as they are done with their work, they will be killed. As soon as they were done with their work, they were killed.