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When you run, your muscles need more energy to fuel the increased movement you are doing. Your muscles get this energy from glucose and oxygen in aerobic respiration and you get oxygen through inhalation when breathing. This means that when your body has greater energy needs, it needs more oxygen and as a result you need to increase your rate of breathing. The more efficiently you can take in air (and therefore oxygen), the more efficiently you can run.

When you are doing an easy run, you shouldn't feel limited by your breathing. You should be able to hold a conversation at your easy running pace - a sign that you are still able to breathe normally and shouldn't have to think too much about how you breathe at this intensity.

As you increase your pace, for example in an interval or tempo run, or even a race, you need to make sure you are breathing optimally. The key here is to both breathe in and breathe out through the mouth. You should ensure you are taking big deep breaths all the way to and from your belly.

As you run faster, your breathing rate will increase, however in order to maximise the volume of oxygen that we get into the body, you should try to continue to take deep breaths filling your lungs all the way up before exhaling again.

Nasal Breathing is one training technique to improve your breathing, whereby you exclusively breathe through your nose, whilst keeping your mouth shut. The purpose of this is to restrict the volume of oxygen that enters the body, which forces the body to adapt and become more efficient at using oxygen and coping with a relative oxygen deficit. Whilst we wouldn't advise nasal breathing to most runners, there is some logic to restricting your oxygen intake. Keep this to your easy runs either way, as when you are doing your harder sessions, you want maximal oxygen intake so that you can run as fast and hard as possible.

Hyperventilating is when your breathing rate speeds up and your breath starts shallowing - this is something that you want to avoid at all costs. Let your breathing speed up as you work harder but make sure to use the depths of your lungs to be able to take that oxygen from the air to keep powering you forwards.

VO2 Max is a term that is often referred to when talking about lung capacity and is commonly used as a measure of fitness. VO2 Max simply means the maximum amount of oxygen that an individual can utilise during intense or maximal exercise. Over time as you get fitter, you'll see your VO2 Max increase, meaning you're able to benefit more from each breath.

Many fitness watches give an estimate of VO2 Max which is a good measure of if your fitness is increasing or decreasing. While the trend may be a good indicator, the raw value is often inaccurate and so you shouldn't compare raw numbers between different devices or uses. An accurate VO2 Max requires an accurate laboratory test which involves running on a treadmill at an intensity that increases every few minutes until exhaustion. You do this whilst wearing a mask which can measure the exact volumes and concentrations of different chemicals in the inhaled and expired air.
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When you run fast or exert yourself physically, things are likely going to get uncomfortable. When you're doing maximal intervals, a challenging tempo session or even a race, you are probably going to get very short of breath and this is not a problem for most runners - it's common for runners to finish out of breath and only by embracing this does it become more comfortable. As you practice this, you'll feel more normal in these sessions and your fitness will gradually improve too - good luck!