I thought I'd say something about the present perfect tense because I have noticed how many students have difficulty in using it properly. This is when everyone groans 'not the present perfect tense again!' I know you know the grammatical rules! I know you know that you have to use 'have' and the past participle. However even though many other languages build up tenses in a similar way it does not mean that the English present perfect is used in the same way. There are even differences in how Americans and British use the tense system! Americans are more likely to use the past simple tense in the question 'Did you eat yet? whereas Brits would say 'Have you eaten yet?' When I meet a new class I often ask students to find out about each other and I have heard a lot of sentences like 'How long are you in Plymouth?' and 'How long are you living here?' from students whose English is pretty fluent. The correct questions are How long have you been here? and How long have you lived here? (Or How long have you been living here?) If you look in a grammar book it may not help you much - the explanations are often so difficult and long that you close the book feeling completely fed up long before you have finished the section! You need something clear and simple to explain just the main uses and I have a link which does that for you. This simple explanation and accompanying online exercises come from the English Language Centre at the University of Victoria in Canada who have lots of great online stuff here.