Try using examples in a context which might be familiar to your pupil:
Since normally says that something HAS NOT happened in a time span which is measured from a certain date. That is, it describes something that happened in the past. eg. There has not been a famine in the Country SINCE 1980.
Until normally says that nothing WILL happen before a pre-set time, or an pre-set event occurs, in the future. eg. The government will not start supplying free food UNTIL everyone is starving.
For is more difficult because it has many meanings, but in the context of the other two words 'since' and 'until', it means something happened 'during' a time span, eg. the famine lasted FOR 2 months.
By is also more difficult because it has many meanings, but in the context of the other three words, it describes the latest time when something is DUE to happen. eg. the government said it will start giving out free food BY the 31st of March.