Driving manual transmission on highway?

Depends a lot on the car and how fast you want to accelerate again. A car with plenty of torque will pull away from 30mph in top gear, although maybe not very quickly. But it's also fine to drop to 4th or even 3rd if you want some brisker acceleration. Of course take care because if they have slowed to 30mph once it suggests stop go traffic ahead, and they are quite likely to slow down again. So gentle acceleration and letting a bit of a gap open up in front is a safer option.

You should always use first to move off from a stop. Using 2nd causes a lot of extra wear on the clutch. Once you are rolling (5-10 mph) you can quickly change to 2nd of course

I have had cars that you always had to have the transmission in a gear that provided the most torque without over reving just to keep up with traffic and others that you could start out from a dead stop in top gear and still keep up with the majority of the traffic. the first one was a fiat X/19 the other was a 1969 Plymouth GTX 440 six pack. If you have a powerful enough engine you can start in any gear

There may be a book in the glove box called an owners manual take it out and read it. It is a wealth of information. It will tell you when and at what speed you should shift. If there is no owners manual in the glove box do a search for it online.

Gears are linked with torque ratio.

Each gear also has a range (mph) that should be used.

Check your manual for the specifics.

Take off in 1st? Yep. Try taking off in 2d or 3d and you'll understand why.

3rd might work at 30 mph for your vehicle. I do not know. Chances are 4th would be better.

Dropping to 30, you should drop down a gear or two. That low in fifth will cause the engine to lug and repeated attempts can cause engine/transmission damage as it is a lot of stress on the engine.

First gear is meant to be used to start the car moving from a dead stop. The rest of the time, at low speeds, you can drop to as low as second gear to keep pace with traffic or other driving conditions.

You need to know your car. I have a diesel that can go down to 30 in 6th with no problem because it has a lot of torque. If your engine starts to judder you need to change down.

Never start off in 2nd. You'll wreck your clutch.

To be safe down shift from 5th to 4th and do not give the car gas, that is the proper way to slow down if a car in front is slowing down, do that or put the car into neutral and let it coast, while your foot covers the brake

You should downshift so you have power when you need it.

Yes it is bad to start in 2nd. That is why there is a 1st.

im thinking you have never driven before because if had, you would not be asking these kind of questions