1) say, tell, talk, speak
1. People often (say/tell) me about their problems.
2. Gwen doesn't (talk/speak) German very well.
3. Mr Brown was (speaking/talking) in front of the school for half an hour.
4. My classmates and I often (talk/tell) about our favourite TV shows.
5. Never (tell/say) lies.
6. Try to (say/tell) the truth.
7. I don't like physics very much. Everybody (says/tells) it is a difficult subject, and I agree.

People often tell me about their problems.
Здесь нужен глагол tell, потому что конструкция «tell somebody about something» означает «рассказывать кому-то о чём-то». После него обязательно есть объект — me.

Gwen doesn't speak German very well.
С языками используется глагол speak: speak German, speak English и т.д.

Mr Brown was speaking in front of the school for half an hour.
В значении «выступать с речью» используется speak. Поэтому правильно was speaking.

My classmates and I often talk about our favourite TV shows.
С выражением talk about всегда используется talk, потому что речь идёт о беседе на какую-то тему.

Never tell lies.
Устойчивое выражение — tell lies («говорить/рассказывать ложь»).

Try to tell the truth.
Также фиксированное выражение — tell the truth («говорить правду»).

I don't like physics very much. Everybody says it is a difficult subject, and I agree.
Здесь используется say без дополнения «кому»: people say that… → everybody says it is….