The first phrase is nonsense colloquially; however, the remaining are proper in terms of their grammatical structures.
Now--for your question and the reason I'm(not I'am) typing this--English is the pinnacle of straightforwardness, conciseness, resourcefulness and, most significantly, unlike most languages, exactness, which means the precision of the meaning of information being conveyed. This would explain our tens of handy synonyms for about every word one may encounter, which are actually very practical and do benefit us, users, since they aren't quite equivalent in definition and therefore provide us with a varied vocabulary with tons of stored methods of sending information, whether vocally or written--our verb conjugations are selfsame, and neither French nor German have in them officially-recognized continuous verb tenses(I am coming; I was coming; I have been coming; I would have been coming; I had been coming; I will have been coming)(all that end with "ing"), all which are not seen in the vast majority of languages, sadly. Nor do the mentioned languages have ways of describing habits or emphasizing phrases without adding whole clauses, whereas in English there's always "I come" (habitual) and "I do come" (emphatic) at one's disposal.
Excluding the first, I will define each of your phrases and try to teach you the distinctions:
"I find that" is in present tense, but English tends to contradict this idea, because what it really means is that you USUALLY(COMMONLY) find that, as if finding were a habit.
"I found that" shouldn't cause confusion, because English doesn't warp the concept of past tense; this conjugation is denoting the past, not the future. So it means you found that BEFOREHAND(PRIOR TO, PREVIOUSLY), as it does in any language, like your mother tongue.
"I have found that" is the trickiest for a non-English speaker to understand. It means that you (have), at this moment, right now, presently, found (something). But it can, too, mean that you found something over a period of time, after an "inspection" which took time.
Sometimes, English speakers use the past tense form to express what has just happened, when a perfect tense is supposed used to fulfil the relatively simple standards of English grammar, which is more versatile than that of other languages I've acquired.
"I have been shown" implies that you were once(at a point in time) shown (something), or that you have JUST(NOW) been shown (something)--please note that I used the adverbs just and now, and many others in my above explanations, to up your level of understanding, and for you to realize that instead of using these words, you should use the VERB TENSES provided.
Please ask if you need more help. And I'm sorry if this reply seems too verbose, or looks like jargon, but it's the simplest I could get--not to be rude.
I'm a 16-year-old boy who has been studying and admiring all about English for well over a year already. I would deeply regret your decision to withdraw yourself from the study of English, because it truly is a wonderful, and fun, language with which, I believe, everybody should acquaint themselves, for obvious reasons and benefits.