MODAL VERBS SHOULDN'T HAVE A PREPOSITION AFTER THEM:

In our Skype English classes many students incorrectly say ‘he can to come at 8’ or ‘they must to go running this evening’. However, all modals except for ‘ought to’ are followed by the base form of a verb (or infinitive) with no preposition after it. Some examples:

• I should go to visit them.
• I think it will rain tomorrow.
• Things could have been better.
• Someone may have seen them.
• They might be home tomorrow.
• They may be late.
• They would have known if they had gone to the seminar.
• He can come by anytime.
• You must get that project finished.
• I shall hand-in the paper on time.

Remember that modal verbs can’t be switched from tense to tense like normal verbs. For example, we cannot say ‘we musted do the homework’ or ‘I will must to study Norwegian’.

Also, modal verbs cannot appear together in the same sentence. Many people say ‘I will can go out tonight’. This is very incorrect. Since ‘will’ and ‘can’ are both modal verbs we need to use ‘to be able to’ after ‘will’. So the correct sentence would be ‘I will be able to go out tonight’. This would translate as ‘Yo podré salir esta noche’.
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DIFFERENCE BETWEEN AFTER AND AFTERWARDS:

The English word ‘after’ is used in the sense of ‘when something or someone has happened or will happen’ as in the sentences:
1. They will arrive after 18h.
2. She will get his chance after Melanie gets her opportunity.

In both sentences you can find that the word ‘after’ is used in the sense of ‘when something has happened or will happen or if someone has done or will do something’ and thus the meaning of the first sentence would be ‘They will arrive once 18h will happen/has happened’. The meaning of the second sentence could be ‘she will get his chance once Melanie’s opportunity has happened’.
However, the word ‘afterwards’ means ‘after that’ or ‘after something happens’ but it is used when no object follows. For example:
1. The concert finishes at 17h. I will arrive afterwards (after that/after ‘the concert’ happens).
2. She forgot to arrive on time. She made an apology afterwards (after that/after ‘her unpunctuality’ happens).
Remember that ‘afterwards’ cannot take an object, ever! So we cannot say ‘I will see you afterwards the concert’ or ‘she will have lunch afterwards giving the speech’. In these cases we’d use ‘after’, thus:
‘I will see you after the concert’ or ‘she will have lunch after giving the speech’.
Unfortunately, with native speakers, there is no doubt that many speakers readily also use after without an object too. So some people will say ‘I will arrive after’ or ‘she made an apology after’. We would say that it’s more correct to use ‘afterwards’ here.
http://www.clasesinglesonline.com