One of the things that makes English difficult to understand for foreign students is its particular rhythm. Many languages are syllabic, that means that every syllable takes the same amount of time. In some languages stressed syllables take longer than unstressed syllables, but still, the pronunciation unit is the syllable.

English is different, we don’t care about syllables, we don’t even care about words, it’s all about beats (sound units). Every beat takes the same amount of time to pronounce. A beat may have one syllable, ten syllables, one word or five words, but it still takes the same a mount of time. At a normal speed, every beat takes about half a second or one second to pronounce, and it doesn’t matter how many things are in that beat. That means that the more syllables we have in a beat, the faster we have to pronounce them, because the global time is not going to change.

But not all words are affected in the same way. There are two different kinds of words in English (as in most languages):

1- Content words the words with a meaning (nouns, verbs, adjectives, adverbs)
2- Grammatical words words with no meaning, but they build the grammatical structure of the sentence (prepositions, articles, verb particles, pronouns, etc.)

Every sound unit (beat) has a content word, which is the most important part of it. You can also have more than one content word, for instance, a noun with an adjective. Then, optionally, we may have one or more grammatical words (structure words). But we always have about one or half a second to pronounce it all, so, obviously, we give more time to the important things (content words) and less time to the grammatical words. So in English, grammatical words are usually pronounced very fast and very weakly (they are often pronounced with a schwa); so weak that foreign students often can’t hear them because they are not used to this particular rhythm of the English language, but if you pay attention, they’re all there, just get used to it!

Here’s an example:

Hi, What were you doing when I rang an hour ago?

Content words hi, doing, rang, hour
Structure words what, were, you, when, I, an, ago
4 Beats (stress marked in capital letters)    HI / what were you DOing / when i RANG / an HOUR ago

So, to pronounce the first beat (hi), we take the same amount of time as to pronounce any other beat, for example the second one:
Hi (1 second) = what were you doing (1 second)

Yeah, on the video they always speak about half a second, but that’s if you speak fast. (Americans often do). At a more normal speed, 1 second is fine. Or if you speak slowly, that’s ok. The important thing is not how fast you speak, the important thing is that every beat takes the same amount of time, so if you take 3 seconds to say "hi", then you should take 3 seconds to say "what were you doing". Well, at least you must try. To practise, you can mark the rhythm with your hand or foot as you read, making sure you keep the rhythm, as if you were singing. Rap music is just that, speaking with rhythm (but a bit more exaggerated). So if you want to speak good English, rap it!

The other important thing is that in English, stressed syllables are pronounced strongly, and unstressed syllables are pronounced weakly (grammatical words are even weaker), so if your listening is not very good yet, maybe the only parts you can hear clearly are the stressed syllables in the key words. But don’t worry, the more you listen to English, the more your ear will get used to this English rhythm. There are no magic recipes, only practise. Listen, listen, listen and listen; your brain will do the rest.