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enter image description here Can you please explain me how can 'shone' be after 'when' ? Or another example. When added to a liquid,antifreeze lowers the freezing temperature of that liquid'

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    It is a subordinate clause in the passive voice, a shortened version of "When (it is) shone through a prism, a beam of white light beaks..."
    – WS2
    Mar 6, 2019 at 11:40

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This is best understood as ellipsis - when shone is essentially a shortened version of when it is shone, with the pronoun referring to the subject of the principle verb. You simply need to insert the appropriate pronoun and form of to be. For example:

When stood straight, I am 5'11" tall.
When I am stood straight, I am 5'11" tall.

When hungry, the dinosaur could eat twice its own bodyweight in a day.
When it was hungry, the dinosaur could eat twice its own bodyweight in a day.

When angry, Bob had a mad gleam in his eyes.
When he was angry, Bob had a mad gleam in his eyes.

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Not simple past but the past participle.

Compare

When eaten too quickly, ice-cream can give you "brain-freeze".

With many verbs the simple past and past participle look the same.

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"Shone" here is not being used as the simple past, but rather as a perfect participle.

It shines through a prism, a beam of light breaks ...

is wrong because "when" is missing. Without the "when," the intended connection between event and result is not specified. So

It is shone through a prism, a beam of light breaks...

is wrong for the same reason.

When shines through a prism, a beam of light breaks ...

is wrong because "when" coupled with a verb forms a subordinate clause that requires a subject. We see the correct form in

When it is shone through a prism, a beam of light breaks ...

Here we have a subject, the pronoun "it" referring to "beam of light," and a verb in the present tense, namely "is shone." Verbs in the passive voice are constructed through some form of the verb "be" used as a modal plus the perfect participle of substantive verb. In that case, the tense is indicated by the modal. So "is" creates a present tense for the verb in the passive voice.

However

When shone through a prism, a beam of light breaks ...

also is correct because the participle "shone" is being used as an adjective to modify "beam of light." (Alternatively, as pointed out in another answer, it can also be interpreted as an ellipsis.)

The past perfect participle for most verbs can be used by itself to form the simple past in the active voice, with a modal to form verbs in the passive voice for any tense, and as an adjective.

EDIT: There are two subtleties that I have somewhat skipped over. One is why the passive voice is somewhat preferable to the active voice in this particular case. The other is the difficulty of explaining when an ellipsis is permitted in English. That latter reason is why I prefer interpreting the correct answer as the adjectival use of the participle. But that is a matter of pedagogical preference, nothing more.

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  • I think you meant "shone" instead of "shown" - first word and others ;)
    – virolino
    Mar 6, 2019 at 13:49

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