1) he has just had lunch. ответ : when did he have lunch ? 2) two new contracts have been signed this week.ответ : when were two new contracts signed? 3) the delegates have been here for 10 days. ответ: how long have the delegates been here? почему в первом
предложении вместо has употр did ? и во 2 предложении вместо have were а в 3 have

  потому что в вопросе, который начинается со слова when всегда глагол ставится в past simple. в первом предложении действительный залог, значит глагол did. во втором предложении страдательный залог, значит глагол were (was)если в предложении есть сигнальные слова just, already, not yet, this week, this year, today, то глагол ставим в форму present perfect -  have (has) + глагол в iii форме.например: when did he come? - past simple                          he has just come. - present perfect

I must wash hands before meal.
Friends must help friends.
People must follow the laws.
You must to do your homework.
I must not to lie.

I would like to talk a little bit about the significance of the news in our daily life. In my opinion, it is extremely important nowadays to keep abreast of local and international news if we do not want to miss an important information that can change our lives. Many news is provided to us through television and the Internet making us aware of all current events happening throughout the world at the moment and steering us a certain way of thinking of them.
          When it comes to a source of information to trust to, I give my preference to the website www.censor.net. There are lots of articles on different topics there revealing the most interesting and discussed information for the day. The website operates on a daily basis, so all news represented there are fresh and timely.
        The most amazing thing about this website is that some local news is also covered, which is unbelievably important for me as I do not need to go to other sites seeking out for information on current issues happening in my native place. 
         The site is structured in a comfortable way for reading, - all main news connected with worldwide affairs are placed in the centre of the page,  while the news that is linked to local events, sporting and business news is situated on both sides of the website`s page. 
      The number of subscribers to the site is on the constant increase and now it approaches a million persons, which makes me feel proud of being one of them.