Here are the following sentences that I am having some problems with. The problem is that whether they are grammatical or ungrammatical.
The speed limit is being introduced gradually.
The speed limit is to be introduced gradually.
The speed limit will be introduced gradually.
To me, all these sentences are grammatical, although the aspects express slight differences. I'm not an expert but if I'm not mistaken, these sentences are syntactically correct?
The last two are in future tense. The first sentence refers to on ongoing process (as of yet incomplete) in present tense.
These are all grammatical, but you are correct that they express slight differences in meaning.
The speed limit is being introduced gradually.
Is being is present continuous. This can be used for an action that is still in process, or it can be used for an action that will be performed in the immediate future. So the introduction has either already started or is just about to start.
The speed limit is to be introduced gradually.
Is to be is a more formal-sounding construction that is either used in commands, or for future events. It's possible that the speaker is instructing someone on how to introduce the limit, or it could just be a statement that the introduction is planned to start at some future point.
The speed limit will be introduced gradually.
Will be is plain old simple future. In terms of tone, it's in between sentence 1 (casual) and sentence 2 (formal); however it's also slightly more concrete than sentence 2. Technically, sentence 2 is only saying that the introduction is planned to occur in the future; sentence 3 is more definite on the fact that it will be done in the future.
See here for a brief summary of different ways English handles present tense.
