Yes, they are correct.

Tienes problemas para pronunciar el "th"? 1) Pon tu lengua detrás de los dientes del frente, tocando sus fondos. 2) Déjalo así y respira. Para hacer el "ough", haz retroceder la lengua y di "a". Y ya sabes pronunciar el 't'. Espero que te sirva.

A few sentences. Are there any mistakes?

A description of a scene that (optional) I saw on TV.

There is a group of quite young people who are quite young - almost as many men as women. They all seems quite sportive and in a good shape.
They're walking in the grass and there are some trees there.

Explanations:

You misspelled "description". Remember: if the verb ends in 'scribe', it becomes 'scription' when it is made a noun. Ex. "prescribe" > "prescription", "subscribe" > "subscription", et cetera.

Although you can omit "that" and still be understood, sometimes "that" stays in the sentence for clarity.

"Is" refers to "a group", not "young people". "of quite young people" is a prepositional phrase; it describes the noun: "group".

I changed this to make it sound better. I don't think it's incorrect, thought.

This construction goes "as ___ as ___".

"They" is plural, so "seem" is used.

"in good shape" is an expression that does not require "a".

"They walk" implies that the people walk routinely. "They are walking" makes it clear that you're only referring to the current moment.

"that place" suggests that you mentioned the place that they are in, but you didn't. "there" is a better choice.

ESTAR "Estar" is used to express that something is temporary (location, condition, etc). It can easily change, and it usually does. "Estoy feliz." - "I'm happy." The speaker describes his/herself as happy. In this sentence, "happy" is the emotion the speaker feels. Emotions are temporary, and be described as a status. This is subject to change; the speaker is happy now, but yesterday they were quite angry or tomorrow they may feel sad. Therefore, "estar" is used. SER "Ser" is used for defining traits (profession, characteristics/qualities, nationality, time, etc). Do NOT think of it as permanent; it may be sometimes but not all the time. "Soy feliz." - "I'm happy." The translation is identical to the previous one, but it has a significantly different meaning. Thinking of the use of "ser", you can guess what this one means. One of the uses for "ser" is for expressing characteristics (like personality traits or physical characteristics). "Happy" acts as a personality trait in this sentence; he/she is generally happy. So for clarity, one could translate it as "I'm a happy person" or "I'm happy in general". SER OR ESTAR? When deciding which to use, ask yourself: "does the description indicate state (mood, current situation, etc)?" If so, use "estar". If not, chances are you should use "ser". Also, remember that although things like profession or religion CAN change, they always use "ser", because they identify/define a person and they aren't variable like emotions or locations.

PRETERITE You use the preterite when the action has a clear duration. "Llovió toda la semana." - "It rained all week." In this example, the preterite for "llover" is used because the raining had a clear duration: all week. "Miré la lluvia por un momento.." - I watched the rain for a moment.. In this example, the preterite is used because the speaker identified the amount of time in which he/she was watching the rain: a moment. It was a very short amount of time. The preterite tells you it was too short to have to be addressed. IMPERFECT You use the imperfect when the action does NOT have a clear duration. This means that there is missing information; the time it started or the time it ended is unidentified. Both may be unidentified. All that the imperfect tense tells you for certain is that it happened in the past. "Miraba la lluvia por un momento.." - "I was watching the rain for a moment.." This example is nearly identical to the previous one, except that this one uses the subjunctive. In this sentence, the speaker is essentially expressing that they were watching the rain for a moment, but they are not sure as to when they started watching the rain in the first place. This suggests that the speaker lost track of time; they sense that it has been a short while, but they really aren't sure because they forgot when it started. It also can suggest that something interrupted their watching the rain (ex. "I was watching the rain, but then I heard a loud noise.") "Miraba la lluvia mucho." - "I watched the rain a lot." This example refers to an indefinite time period in the past. It could be the course of days, weeks, or months. They may or may not watch the rain anymore. PRETERITE OR IMPERFECT? If you don't know which to use, ask yourself: "do I have a fair idea as to when the action started and ended?" If so, use the preterite. If not, use the subjunctive. Context helps. I like to think of it this way: preterite = event, subjunctive = process

I think you accidentally swapped English and Russian. And I'm in no place to give you pointers (because I know zero Russian), so I'm going to do a quick search. .. Okay, apparently it's called a "voiceless alveolar affricate", and it's pronounced similar to the "zz" in "pizza" and the "ts" in "cats", just like you said. So maybe your tongue is in the right spot but you have to put it a little lower? I don't know. Oh, I found this: http://www.study-languages-online.com/russian-articulation.html It offers a sound file, a graphic (showing you how your tongue should be positioned and everything), and a description for each sound.

To add to what Sergio said, I think people also may use it when they're announcing their entrance/presence. Another close English equivalent besides "Here it is.." could be "Behold!", but not so dramatic. I may be wrong, though.

Like everyone else is saying, it doesn't sound wrong, but there are more natural ways to say the same thing. Also, the correct spelling is "responsibility". But you seem to know that anyway, since you spelled it right your description.