Future forms - which tense is good?

Question

I am very confused using future tenses to described situation will be completed at some time in the future.

Let's say that I see that workers are building a highway and I can say to a friend:

1. The highway will be finished next year.

2. The highway is going to be finished next year.

3. The highway will have been finished next year.

or let's say I am repairing my car and somebody asked me when I'll finish. How can I answer?

1. I think it'll be finished soon

2. I think it'll be ready soon

3. I think it will have been finished soon

4. I think it is going to be finished soon

etc

Answer 1

In general, if you are referring to a single, simple subject, like "the highway" and "being finished", it's usually easiest and most correct to use the simple future, which is just "will X" or "is going to X". For your first example question, either of the first two options is good.

The highway will be finished next year.
The highway is going to be finished next year.

They mean essentially the same thing: It will be finished. When? Next year. The third option,

The highway will have been finished next year

uses the future perfect (will have Xed), but it doesn't quite make sense. We use the future perfect to relate one thing happening in the future to another specific event in the future, but there is only one thing happening: the highway being finished. Saying will have been finished means that the highway will have already been completed, so it only makes sense in relation to something else. You would use the future perfect to say something like

The highway will have been finished by the time the President visits next year

For your second example, the most idiomatic answers, to an American English speaker, would be

I think it'll be finished soon
I think it'll be ready soon

The fourth one,

I think it is going to be finished soon

is grammatically correct, but it sounds a little stilted to this native US English speaker.

The third one,

I think it will have been finished soon

sounds strange, because, as with the sentence about the highway, it sounds like you're relating the finishing to a future event...but there is no future event!

References:
Future Simple and Future Perfect
Future Perfect Simple
Future Perfect

Answer 2

All of them are correct, although some are more common or unusual.

The highway will be finished next year: This is the simplest and most common.

The highway is going to be finished next year: This is fine and reasonably common.

The highway will have been finished next year: This is correct, but unusual. You are specifically placing the listener in a mental time frame after completion. For example: We are opening an office building. Highway construction is causing bad traffic. Will we have a traffic problem for our new office? No, the highway will have been finished by the time we open the new office. Mentally, we are seeing the day the office building opens. There is no traffic in our metal image, because the highway is already finished.

I think it'll be finished soon / I think it'll be ready soon / I think it is going to be finished soon: These are all common. Note that "it'll be" is a contraction for "will be", and "will be" is effectively identical to "going to be". Any page I've seen trying to explain some difference between those two phrases is empty babble.

I think it will have been finished soon: This is valid, but unusual. It extremely unusual to combine it with "soon". You are mentally placing the completion of the car in the past. Here's an attempt at an example: Why are you rushing to paint the car? I am going to drive across the country in 8 months. At that point it will have been finished for over half a year, and it will have begun to rust. Note that I mentally placed myself 8 months in the future, at the time when I drive across the country. I am picturing myself looking back in time. I am picturing that the car was finished more than half a year ago. I am picturing that half a year of rust has grown on the car. We are talking about "the past" from that future viewpoint. I'm painting the car now because I see myself in the future wishing that I had painted the car, to prevent the rust.