I don't have a measuring glass. I just have a tablespoon (15 ml). Can 1 tea spoon be considered half of 1 table spoon?
One tablespoon is three teaspoons, so no, it can't be considered half a tablespoon because it's one-third instead :)
You can use three teaspoons to measure out one tablespoon, but it might be tricky to measure the other way: as Rumstacio said below, the 1/3 is by volume, and it can be difficult to eyeball the volume of a semi-sphere (1.3 the height of the spoon is not 1/3 the volume because it's wider at the top).