[anw]

In college, my Japanese teacher told us that a lot of foreigners get these characters wrong, but if you know the stroke order then it's easy to see the difference.

シ (shi) is written top to bottom. You can see that all the starting points for the strokes line up vertically on the left. Also, the last stroke curves from the bottom-left to the upper-right.

ツ (tsu) is written from left to right. You can see that all the starting points for the strokes line up horizontally at the top. Also, the last stroke curves from the upper-right down to the bottom-left.

ン (n) lke 'shi' is written top to bottom. The starting points for the strokes line up vertically on the left. It also uses the same direction for the longer, final stroke as 'shi'.

ソ (so) like 'tsu' is written left to right. The starting points for the strokes line up vertically on the left. It also uses the same direction for the longer, final stroke as 'tsu'.

[anw]

Replying since I can't edit: The sentence for "ソ (so)" should read "The starting points for the strokes line up horizontally at the top".