Когда используются артикли? не в какой момент используется the или a, а когда они используются в принципе. объясните, , в том числе на примере "i'm very good in _ bed" и "a sleeping couple is lying in _a_ bed".

Артикли делиться на два
1)a,an неопределенный
2) the определенный

Артикль a используется перед словами, которые начинаются с согласного звука, а артикль an — с гласного.
Ну примеру::
A book — слово начинается с согласного звука.
An apple — слово начинается с гласного звука. Есле будет An book оно не красива звучит.
THE
2) I’d like to buy a dress. — Я хочу купить платье.
Речь идет не о конкретном платье, а о каком-то платье.

А если бы вы сказали:
I’d like to buy the dress — это означало бы, что вы имеет в виду не какое-то неизвестное платье, а конкретное платье, вот это.

Это классический пример: при первом упоминании используется артикль a, при повторном — артикль the.
АртикльПеред словами, которые начинаются со звука:Ab, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y, z
An a, e, i, o, u
Все понятно??

School uniform is an integral part of school life in many countries. Most schools in Russia also require school uniforms. However, some school accept freestyle outfit. There are different opinions on how the standard school forms should look like. Some schools have dark blue uniforms, while others use dark red or dark green colours. Some schools in Russia accept white shirts and black skirts for girls and black trousers for boys. Nevertheless, all these school uniforms have something in common. They all look rather smart and strict. Children’s opinions on school uniform differ. Some say they like wearing it, while others prefer to wear a pair of jeans and a T-shirt. In my opinion, the school uniform has both advantages and disadvantages. First of all when children wear a school uniform, they realize that they are at school not at a swimming-pool or stadium. It adjusts them to school atmosphere and helps to study harder. Secondly, school uniforms prevent social inequality. It means that both rich and poor children wear the same clothes. Furthermore, children don’t have such problem as what to wear in the morning. The disadvantages of school uniform include the loss of individuality. Everyone becomes alike and children look similar. The second problem may be untidiness. As children wear the same clothes every day their uniform starts to look a bit untidy and faded. Last but not the least school uniforms have become rather expensive lately. So, not every parent wants to buy it. As for me, I support the idea of wearing a school uniform. I think that at home we can put on what we want, but at school we need to follow the rules.

1. Raising (Participle I, обстоятельство) the output we must not forget the task of improving (Gerund, определение)  the quality at the same time.
2. Engineers find use for titanium and prefer it to aluminium which loses its strength rapidly when subjected (Participle II, обстоятельство) to high temperatures.
3. The Moon, like the Earth, shines with the reflected (Participle II, определение) light of the Sun.
4. All the difficulties in organizing (Gerund, предложное косвенное дополнение) the laboratories and in supplying (Gerund, предложное косвенное дополнение) them with the necessary equipment had to be overcome.