Hi Karina,
As a general rule a line that comes up in the TEST windw within the timeframe of the test is a positive.
An evaporation line is a line that appears where the test line would be after the time frame for the test. Evaporation lines are greyish and not coloured. So if your test says to read the results at 5 minutes a feint line in the test window at the 5 minute mark is likely to be a feint positive. Sometimes if you test a little early and there isn't a very high amount of hcg in your urine the test will not show a prominent positve line.

However if this same test looks negative at 5 minutes and then you go back 10 minutes later and there is feint line that is greyish this line is probably an evaporation line...

If you have a query over your test it would be best to wait 48 hours and test again using first morning urine.

Goodluck