There is nothing wrong .You are really hard to learn Chinese.In actually,it's a natural behavior in our daily conversations,it may be hard for you to learn, you will understand it when you have the language sense.Do not worry. Ps:I'm not good at English,I would appreciate it if you can piont out some mistakes of me. I can also help you with your chinese.

1) Third tones change to the second when it is third tone character right after them. 2) Rules for tones changing in "一": - The first tone: yī, when using solely, or using at the end of words, or showing ordinal and cardinal numbers. E.G. 数一数二 说一不二 一楼 统一 万一 - The second tone: yí, when it is in the front of the 4th tone characters. E.G. 一致 一阵 一切 一块 - The fourth tone: yì, when it is in the front of characters which are not with 4th tone. E.G. 一杯 一直 一早 - Toneless: yi, when it is in the characters between. E.G. 看一看 试一试 等一等 3) Rules for tones changing in "不": - The fourth tone: bù, when using solely, at the end of words and phrases, and in the front of characters which are not with 4th tone. E.G. 不，我偏不！ 谁敢说不？ 不通 不巧 -The second tone: bú, when it is in the front of the 4th tone characters. E.G. 不是 不但 不要 不好 不在 -Toneless: bu, when it is in the characters between. E.G. 了不起 吃不了 去不去 I hope my answers help! Wish you all the best with your studies. :)

You are totally correct. For the consecutive 3rd tones rule, I personally find it really hard to pronounce 2 consecutive 3rd tones, and I think that's why we change the first one to a second tone. You don't have to pay special attention to it. Even if you pronounce it as 3rd tone, it will somehow sound like 2nd tone. You can record your voice to check it out. For 一 and 不，I think the only way is to memorize the whole phrase. Actually, no Chinese think about the rules before they speak. Just get used to it. =)

Helpful link, thanks Jimmy.

This is called tone sandhi. Relevant wikipedia page https://en.wikipedia.org/wiki/Standard_Chinese_phonology#Tone_sandhi Hope it helps.