- In order [ for food to be considered raw ], it must not have been heated over 46C.
- [ For a person to be considered a suspect case of Ebola ], they must have a sudden fever.
- [ For a laptop to be considered one of the best ], it'd need one of these.
In the above three examples, the italicized expressions (which are also marked by the pair of […] brackets) are to-infinitival clauses. These to-infinitival clauses have their own subjects which are marked by the word "for" which is also a clause subordinator.
Question #1: The structure "For ... to be considered …" is used in all the above sentences. I think at least in the last two sentences, it could be said "A person/laptop is considered ..." instead. Am I right? or would they imply different meanings?
Answer: In general, you can't blindly replace the to-infinitival clause with a finite clause and expect the result to be grammatical. Though, sometimes it will result in being grammatical, very often it won't. The basic meaning of the to-infinitival clause, when taken in isolation, can often be roughly paraphrased by a finite clause (though the meaning is not really the same). For instance, using the to-infinitival clause from the OP's example #3:
- For a laptop to be considered one of the best
it could be thought to have, in a rough sense, the meaning of the finite clause "a laptop is considered one of the best" (which was suggested by the OP).
But you can't blindly use that finite clause to replace the to-infinitival clause in example #3:
- 3.b. A laptop is considered one of the best, it'd need one of these.
for the result (#3.b) is ungrammatical. But notice that for example #1 when a finite clause is substituted for its to-infinitival, it seems to produce a somewhat dubious result:
- 1.b. In order that food is considered raw, it must not have been heated over 46C.
Question #2: What grammatical function does the "FOR ..." part play here?
Answer: The word "for" in the to-infinitival clause is a clause subordinator, which is used to mark the subject of the to-infinitival clause.
Aside: In the OP's three examples, the subject of the to-infinitival and the subject of the main clause are the same, and the to-infinitival's subject could be deleted. For instance:
- 1.e. In order [ to be considered raw ], food must not have been heated over 46C.
- 2.e. [ To be considered a suspect case of Ebola ], a person must have a sudden fever.
- 3.e. [ To be considered one of the best ], a laptop would need one of these.
Aside: For example #3 (and #2), notice that the to-infinitival clause could be moved back to its canonical position in the sentence, as seen in version #3.f:
- 3.f. A laptop would need one of these [ (for it) to be considered one of the best ].
and this shows that the difference between version #3.f and the OP's example #3 is that the OP's example had its to-infinitival clause fronted.
Aside: There might be a possibility that one or other of those "for" words in the expression could be considered to be a preposition -- where the expression might be parsed as "for [NP + to-infinitival]", or as "PP + to-infinitival".
Here's one such example, where there could be ambiguity in the parsing:
It would be good [for us to have a period on our own]. -- (infinitival with its own subject)
It would be good [for us] [to have a period on our own]. -- (PP + to-infinitival)
The parse for version #1 has as its infinitival clause the expression "for us to have a period on our own", where the pronoun "us" is the subject of the infinitival clause.
The parse for version #2 has as its infinitival clause the expression "to have a period on our own", while the expression "for us" is a mere prepositional phrase that is not part of the infinitival clause.
This above example is borrowed from CGEL. The ambiguous parsing is explained in CGEL page 1183, footnote 8:
Examples like It would be good for us to have a period on our own are ambiguous according as for us is a PP dependent of good (cf. To have a period on our own would be good for us) or part of the to-infinitival (cf. For us to have a period on our own would be good).
CGEL is the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language.