你才想要这个. (It was you who wanted this).

This is used when contradicting the other person’s opinion.

- 你想要这个吗？
= You want this?
- 你才想要这个。
= YOU want this.

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一会儿 (yi1hui4(2)er) the tone can vary. Some will pronounce huier with a second tone, while others use the fourth tone.

This is all correct except for one thing – some will pronounce it with a fourth tone, and some will use the _third_ tone. Not the second. However it’s often used before “I”, as in 一会儿我就去 (I’ll go in a minute), which makes the third tone sound like a second (one of the three tone-changing rules).

Also, in standard Pinyin notation, a combined “er” sound is always indicated by an additional “r” at the end, in this case, yi huir.

Also note the “yi“ will change into the fourth tone (another of the three tone-changing rules).