If I want to remove the front wheel to lock it with the frame and rear wheel, do I have to disconnect the front brake? The videos I saw all did that, but may I just take out the wheel while keeping the brake pad untouched?

Depends on your wheel. If you can freely remove the wheel without undoing the brakes then by all means do! However you either have VERY skinny tires, or your brakes are not properly adjusted. I know on my 3 bikes I have to undo the brake pads to remove the wheel, even on my 700x40 bike.

Yep, most tires stick out far enough past the braking tracks on the rim that this just isn't going to work.

vol, What kind of brakes? Large tires can be problematic even if there's some fashion of easily spreading the pads.

Brad

If it is a modern bike with cantilever or V- brakes it is easy to open the brakes up-just remove the cable.
Older better quality bikes-maybe have a lever you can flip to open up the brakes a bit and get the wheel out of the calipers..
Now less expensive older caliper brakes bikes-center pull or sidepull- sometimes don't have a lever to loosen the brakes. In this case if your tires are skinny enough you can muscle them in and out-sometimes you have to let some air out which means carry a pump.In the old days-70's-I muscled the wheels in and out- of course I had the brakes adjusted a bit wide(not ideal) to allow this.
There is usually a way to get the wheel off without too much trouble even if it involves having the brakes adj in a less than ideal way-as long as you don't run out of lever travel on max efforts the brakes will work OK.
Charlie

On my bike, I can't even remove the wheel if the V-brakes are disconnected. I need to deflate the tyre first.

On that bicycle, routinely removing the front wheel to lock it to the rear wheel is tire(!)some.

If your bike has an easy way of opening the brakes to remove wheel, then by all means do. If the tire is a little wider then brakes then you may be able to give it a little pop with the open hand to push it thru. The same to put it back in place. If its a skinny enough tire , then none of this should be a problem

Your question answers itself. Try removing the front wheel without opening the brake wider.

If you can do so, then the answer is no.
If not because the tire is too wide, you have to open the brake or deflate the tire to gt the clearance needed, and the answer is yes.

We can't answer for you, you just have to try.

It's Tektro V-brake, 700x35c wheel.

What I wanted to know is, if I undo the brake, could the brake pads become mis-aligned when putting the wheel back?--will I need to readjust the brake or it will be the same as original? (Obviously I have never tried yet.)

It should be fine. Before switching to disc brakes, I removed my front wheel every time I put my bike on the roof rack or 5 times per week for ten years. No adjustment troubles whatsover. Just squeese together and release the noodle. Check this video out. https://www.youtube.com/watch?v=qW9rk...eature=related

I suspect that you're trying too hard.

Look at where the cable attaches to the wheel brake arches. See that little metal noodle thing. Squeeze your brake pads together against the wheel rim and, now that the cable feels loose, pull that metal noodle sideways. It'll come out of the little metal stirrup that holds it. Now the brakes will open wider and allow you to remove the wheel easily.

Whe reeplacing the wheel, simply reverse the process and the brakes will line up exactly as they were before.