I
conjunction
BAD : I do badly in tests for I am lazy.
GOOD : I do badly in tests because I am lazy.
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Do not use for to mean 'because'. This usage has almost disappeared and is found only in very formal styles.
II
preposition
1
BAD : I have come to London for learning English.
GOOD : I have come to London to learn English.
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To explain 'why' someone does something, use a to -infinitive (NOT for ): 'She opened the door to let the cat out.' In formal styles, in order to is also possible: 'UN troops have been sent to the trouble spot in order to restore peace.'
2
BAD : I studied in Freiburg for to be a kindergarden teacher.
GOOD : I studied in Freiburg to be a kindergarden teacher.
BAD : Being married makes it easier for to answer this question.
GOOD : Being married makes it easier to answer this question.
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For never comes immediately in front of a to -infinitive. Compare: 'It will be difficult for John to say no.' 'The suitcase is too heavy for anyone to carry all that way.' ( for + sb/sth + to -infinitive).
3
BAD : Unemployment has become a serious problem for the last few years.
GOOD : Unemployment has become a serious problem over the last few years.
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Use for to say 'how long': 'He was with the company for forty years.' 'He hasn't eaten anything for the last two days.'
Use over/during/in to say 'when', especially when this is a long period of time: 'She's been a great help to me in recent months.' 'During the next ten years he worked his way up from office boy to general manager.'
4
BAD : I'm waiting here for almost an hour.
GOOD : I've been waiting here for almost an hour.
BAD : I didn't do any travelling for the last two years.
GOOD : I haven't done any travelling for the last two years.
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For actions and situations which began in the past and continue up to 'now' (the moment of speaking), use the present perfect tense: 'I've been working part-time for the last six months.' 'We haven't seen each other for the past eight months.'
Note that in informal American English the past tense is also used for this purpose.
See also SINCE 1 (↑ since )