'Macbeth is written by Shakespeare', is a valid, grammatically correct sentence. But curiously it contains the verb 'to be' in the present tense, with a past participle. We can recognise that the sentence is in the passive voice, but what is the tense of the verb(s). Is 'written' working as a verb, or as another part of speech?
Now we can also use 'is' and 'written' in the same sentence, but in separate clauses, such as in:
'Written by Shakespeare, Macbeth is a masterpiece'. How has the role of the verbs changed?
Best Answer
The verb form "written" is a participle that happens to be named past participle. It has no tense: no present tense nor past tense. This also holds for the other participle verb form that happens to be commonly known as the present participle (also known as gerund-participle).
(Aside: It so happens that the past participle could be connected to a secondary past-tense when it is used in a perfect construction. But that is due to the perfect construction.)
If a participle is used in a non-finite construction, then a "tense" could be borrowed from a superordinate clause. The role of the verbs in both of your examples haven't really changed much at all. In both examples, the "tense" of the verb is is being borrowed by the clause that is headed by the verb written when the written clause is being interpreted. (Syntactically there are differences between the two sentences, but I don't think you are asking about those differences.)
Here's a way to parse the two sentences, with a clause in brackets "[ ]", and with the head verb of each clause bolded:
[ Macbeth is [ written by Shakespeare] ].
[ Written by Shakespeare ], [ Macbeth is a masterpiece ].
If you want a traditional grammar type of explanation, well, er, someone else can give you one of those. :)