There are a few katakana and hiragana that are quite similar, and are easily mistaken for one another. In this page I look at the ones that have given me problems, and the ways I have found that helped me best in distinguishing them.
The katakana ン (n) and ソ (so)
These kana are so similar, they are often confused, but believe me when I say after a while you will be able to distinguish them without thinking about it. The encouraging thing is that these are, in my opinion, the hardest of the similar kana to distinguish.
The fundamental difference between them is the alignment of the lines. To picture this, draw a box around both of them, and you'll notice that "ン" both lines touch the left edge, and "ソ" both lines touch the top. You could try linking the letter "o" from "top", and link it with the "o" in "so" without too much of a stretch, as both have the same sound.
The katakana ツ (tsu) and シ (shi)
This one caused me quite a lot of problems at first. One of my first queues for learning it was picturing "シ" as a smiling face, and with the phrase "she (shi) is smiling". The problem I had was that tsu also looks a bit like a smiling face so I looked for another alternative.
When you discover the method of learning it, you also come to realise why it's a good idea to learn the hiragana first, before the katakana. This is because the two dots of the katakana here could be thought of as the straight line in the hiragana. So, if you take つ (tsu) and draw two lines crossing the top line, before erasing it, you end up with "ツ" (tsu). The same system works with し (shi) and シ (shi).
The hiragana ち (chi) and さ (sa)
I'm not sure if this one causes a lot of people problems, but I've found it always takes me a time to tell them apart. Mainly because of how I remembered them doesn't seem to discern the direction of the circle at the bottom. Again, this is probably a little bit down to the way I first memorised them by using mirrors - i.e. I saw there was almost an "S" shape in ち (chi) and a "C" shape is at the bottom of さ (sa). So now, every time I look at the shapes, I see the letter fiirst, and have to tell myself that the one that looks a bit like an "S" isn't "sa" but "chi". I'm not sure of a better way to distinguish these letters, but my system did work, and after a while you don't need to think about it.
The hiragana こ (ko) and the katakana ニ (ni)
This isn't a problem at first, and a lot of the time, as most times you can either distinguish whether katakana or hiragana are being used in the rest of the word. There are a couple of times that I've had a little problem when hiragana and katakana are mixed. I think being aware of this one, and to take care when reading is all that is required here so I did think it worth mentioning them.
Also worth noting is that the katakana ニ (ni) has straighter lines; a trait that generally distinguishes katakana and hiragana. In other words, the katakana tend to be more angular and rudimentary than their hiragana counterparts.