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The biggest thing that's going to make you sound "too polite" is using the -masu form of verbs in places where you're not expected to be humble. It's appropriate to talk like that to strangers, but it's awkward among friends and peers. "desu" isn't quite as bad, in that it's not terribly out of place in casual conversation, but replacing it with "da" or dropping it entirely is more casual. ("da" seems to hint of a little more masculine casual from what I've seen; it seems women are more likely to drop it completely.)

Dropping unnecessary particles will make you sound more casual, but it's not considered part of "polite speech" to include them -- using all of the appropriate particles sounds just fine in casual speech.

Other than that, though, learning to speak casually is a matter of experience. It's like learning to speak casually in your own native language: they don't teach it in school; you have to figure it out by talking to your friends.

It is kind of hard to explain how to speak in proper conversational form (i.e. too long of an explanation), but in essence, understanding conversational form has two parts: 1) being able to understand what is spoken to you (the main thing is the topic of what everyone is talking about) and 2) being able to respond with what you think about said topic. Obviously, #2 is going to be easier than #1 in this case because as was said before, in conversational form, particles are dropped and sentences are shortened in conversation a lot. Sometimes I don't feel like explaining what someone originally said so I may just bring up a sentence that is not quite complete that my Japanese friend will interpret and be reminded that I am talking about something that was previously mentioned in the conversation. The shorting of particles doesn't quite do it, but it is the shortening of subjects and referencing of things that causes confusion both among non-natives and natives of Japanese.

You da(formal) and Mitai(casual) are used when you have direct information about something, and can see, hear/*insert sensory verb here* the object in question, like you see the price of a watch or feel silk sheets for the first time. Using either one asserts you have more of an authority about the subject at hand, and mitai can even be used when you're not sure of what something is like.

Used as an adjective, you na functions as a comparison between the subject and something else, like when we say someone is as dumb as a rock or as gentle like a summer breeze. It's most often seen in this sentence construct:

*x* wa *y* no you na *comparative word*
(X is as *comparative word* as Y)

Rashii is used when the thing your referring to is a bit less solid in your mind, and is culled partially from what you've read or heard about the subject. Also used to expressed when something has the qualities of something else, like when we say someone is tomboyish or manly.

Sou works on two different levels. when it's simply tacked on to the end of a sentence it implies what you're speaking of is based solely off of stuff you've read or heard from others about the subject. when it attaches to the stem form of a verb or adjective(this use doesn't work with nouns), it bases your assertions on visual information you have then and there, like when we look at someone and say "You look ready for this"

Had to add on a bit.

Rashii - It's usually when someone has the qualities of that which they are, not something else- that's mitai.

Ex: Otona rashii. Adult-like (when subject being spoken about is an adult).

Can be used with people too- A-san rashii ("just like A"), A-san rashikunai "Not like A.", but only when the subject IS A.

As Koiyuki said, it can be used when quoting information when the source is not clear, such as gossip: A-san wa kodomo ga dekita rashii (I heard A got pregnant)!

Mitai usualy means that something "seems" A, but is not really A. Otona mitai - Seems adult (insinuated that the person is NOT adult).

Another way to say "Adultlike" would be to add '-ppoi' - otonappoi, "seems adultlike". Doesn't have to be true or false. Onnappoi (girlish, can refer to either gender); kodomoppoi (childish), and so on.

Hope that makes sense. c: I'm not good at explaining things.

I can provide that for you.

Hearsay:
あのゲームは楽しいだそうです。 I heard that game is fun.
スミスさんは朝に走るそうです。I heard that Mr. Smith runs in the morning.

Sensory information:
あのゲームは楽しそうです。 That game looks fun.
スミスさんは走りそうです。It looks like Mr. Smith is running.

As you can see, the two uses are slightly different.

The recipe for hearsay is simply that you take the plain form of the statement you want to make and add "sou desu" / "sou da" / "sou". If you want to attribute the statement to someone, you can say "X ni yoru to ~" at the beginning, which is approximately equivalent to "according to X, ~".

The other form is more of a "seems like" or "looks like" form. You take the stem form instead of the plain form and add "sou" to turn it into a na-adjective, which means you generally put the copula afterward but you can also say 楽しそうなゲーム ("fun-looking game").

1-command form of painting something out(塗りつぶす)

2-formal negative linking version of the word for something growing stale, or fading colors(色あせる)

3-past tense for the gerund form of blocking or sealing something(from the casual version of -ている)(塞ぐ)

4-past tense for the gerund form of spending time doing something(like spending time making a cake or passing time learning something)(過ごす)

5-either the command form of losing something(忘(す)る ) or the stem form of forgetting something (忘れる)

There's something I've always been curious about, concerning Japanese counters. I know well that to express you *verb* a certain amount of something you'd use this sentence formula:

[object]を[number+counter][action]

But is it possible to omit the object and the particle proceeding it, if the context is understood by both parties?