Unit 7bc Book

To describe a principle of a machine, the Present Simple tense is usually used because it describes something which is either repeated or which is always true:

Coils create a magnetic field. They are embedded in the WTMD frame.

This tense was covered in the AEL3 course, but if you feel you need to revise it, look at this grammar overview.

However, in some specific situations, the Present Continuous/Progressive tense can be used. For example, you can use it to distinguish between aspects:

When/After a passenger walks through the metal detector, a magnetic pulse is sent into the opening.

You can also see that this sentence means that the action expressed in the subordinate clause after “when/after” is finished first – before the action expressed in the main clause starts:

Když člověk projde detektorem, do prostoru je vyslán magnetický puls. (přítomný prostý čas pro vyjádření dokonavého vidu, tj. děj vedlejší věty po “when/after” je napřed dokončen, než nastane děj věty hlavní).

vs.

When/While/As a passenger is walking through the metal detector, a magnetic pulse is sent into the opening.

Když člověk prochází detektorem, do prostoru je vyslán magnetický puls. (přítomný průběhový čas pro vyjádření nedokonavého vidu)

You can see that the Present Continuous/Progressive tense is used to talk about a situation in progress (is walking) which is interrupted by a short action (... is sent... – the Present Simple tense). In other words, in one short moment, the two actions happened parallelly. This means that you can also use the Present Continuous/Progressive tense to talk about parallel actions that are in progress at the same time:

When/While/As a passenger is walking through the detector, his/her bag is going through the X-ray machine.

Here, the two parallel actions take some time. This is a rather rare use of the Present Continuous/Progressive tense, as you can see if you compare it with its common uses in this grammar overview.

 However, where a continuous/progressive tense is used very often is when you want to talk about parallel actions happening at the same time in the past. In such a case, the Past Progressive/Continuous tense is used. Look at the sentence below and compare it with the sentence in the Present Progressive/Continuous tense to see the analogy:

 When/While/As a passenger was walking through the detector, his/her bag was going through the X-ray machine.

As you can see, the two actions were happening in parallel for some time. Similarly:

When/While a passenger was walking through the metal detector, a magnetic pulse was sent into the opening.

Here, however, the action in the main clause took just a fraction of a second; therefore, the Past Simple tense is used. These two types of sentence must be distinguished from the one below:

When/After a passenger walked through the metal detector, a magnetic pulse was sent into the opening.

Here, the actions are not parallel, but consequent with the action expressed by the subordinate clause after “when/after” happening first. Therefore, no continuous tense is used, but there is the Past Simple tense in both the clauses. Now, read about the use of the Past Progressive and Past Simple tenses in this grammar overview. After that, see if you understood the overview fully and complete a similar one here. Then practise this grammar point first in the context of airport incidents , then in the context of an electrical accident, and finally in the context of accidental technical discoveries.