おはようございます。 You pronounced おはよございます。よ should be longer(two moras). ※about 'mora', you can see the following. https://en.wikipedia.org/wiki/Mora_%28linguistics%29 アレクサンドラです。 Your accent was wrong(different to standard Tokyo accent). You pronounced ド with high pitch. レクサshould be high. ロシアの Your accent was wrong. You pronounced シアのwith high pitch. ロshould be high. 日本語が Your accent was wrong. You pronounced ほwith high pitch. ほんごがshould be high. べんきょうを Your accent was wrong. You pronounced んきょwith high pitch. んきょうをshould be high. 皆さん、勉強を手伝ってと私の発音をチェックてくだかい。→皆さん、勉強を手伝って、そして私の発音をチェックしてください。→皆さん、私の発音をチェックしてください。　※とis only between nouns. 手伝って Your手伝ってsounds like手伝て. 'っ'［t］of てつだって［tetsudatte］is also one mora. You have to keep a pause between だand て. 発音を Your accent was wrong. You pronounced つおwith high pitch. つおんをshould be high. どうも Your accent was wrong. You pronounced うもwith high pitch. どshould be high. ありがとうございます You pronounced ありがとございます。とshould be longer(two moras). どうもありがとうございます。→よろしくおねがいします。I hope you will take care of me.＝Thanking you in advance. To say どうもありがとうございます is strange because you haven't known whether someone gives you a favor. It might be a correct expression in English. But in Japanese, we don't say "どうもありがとうございますThank you" before we know someone gives us a favor.

must be 私の 発音 を チェックて 下さい

I often get uneasy about my english pronounciation, too. Let's keep studying hard together. Good luck :)

I don't know too much words, but my teacher tell me, I must be sure that I pronounce new words correctly. With all Japanese ”らりるれろ” and "し" and "う" and other... If you understand me it should be almost correct. Thank you, Ryoko.

Of course, we can understand you without any problems! Please don't worry! Besides, you never have to say sorry for your Japanese, we are learners and making mistakes is something necessary. The most important is practicing and learning from mistakes, it's may way of learning language. Good luck with your Japanese!