Когда в языке ставиться a? например i have a а некоторых предложениях оно стояло, а в некоторых нет. с чем это связано?

almost all people are fond of travelling. it is very interesting to see new places, another towns and countries. people may travel either for pleasure or on business. there are various means of travelling. for me there is nothing like travel by air; it is more comfortable, more convenient and, of course, far quicker than any other means. there is none of the dust and dirt of a railway or car journey, none of the trouble of changing from train to steamer and then to another train.

with a train you have speed, comfort and pleasure combined. from the comfortable seat of a railway carriage you have a splendid view of the whole countryside. if you are hungry, you can have a meal in the dining-car; and if a journey is a long one you can have a wonderful bed in a sleeper.

travelling by ship is also very popular now. it is very pleasant to feel the deck of the ship under the feet, to see the rise and fall of the waves, to feel the fresh sea wind blowing in the face and hear the cry of the seagulls.

many people like to travel by car. it is interesting too, because you can see many sights in a short time, you can stop when and where you like, you do not have to buy tickets or carry your suitcases.

a very popular means of travelling is hiking. it is travelling on foot. walking tours are very interesting. hitch-hiking is a very popular method of travelling among young people. but it is not as popular in our country as abroad.

last year my friends and i went to the mountains during our  holidays.    it was amazing. we went skiing and sledding a lot, we were playing snowballs and making a snowman. the weather was perfect: frost and sun, all the time we had to wear  sunglasses as in summer.    in the evening we decided to  have dinner in the hotel restaurant and had fun. after dinner we went to our rooms which  were cosy and bright. so  jacek was  playing the guitar, anna was  singing, i was chatting with my parents, max and alice were watching a funny movie, and martha was playing her favourite computer games.  everybody was relaxing. it was great.  then everybody wanted a drink of tea and veronica brought her thermos. we poured tea in the mugs, the tea was hot and fragrant. but we couldn't drink it, because it was salted!   it turned out that in the morning veronica was afraid to be late, she hurried and confused salt and sugar! everyone laughed. it was really fun.  then we came to the night cafe  on the nineth floor and drank sweet  tea there.  i will remember that salt tea all my life!