Pre= before. It is always said with a heard r and soft or hard E sound. This word can be said with either soft e or E. then an s and then a hard I sound. Finally s. The final letters are like ice. PrE - s - I - s Even if someone said incorrectly as per - s - I - s. (which happens) you still hear the r.

The "r" in precise is always pronounced. Maybe it is pronounced very softly, but it is there. For questions like this, I recommend you go to the Merriam Webster dictionary site (m-w.com). You can look up the word and click on the audio icon to hear the standard American pronunciation.

Look up a term called "schwa" in Wikipedia or somewhere. You're going technical here, yes, that situation happens a lot, and some "pro/pre" situations are perfect examples of it. Like, "professor". That's also partially why most Chinese students' "believe" sounds so unbelievable, just so you know. Look it up, man. It's a phonetics thing. Edit Thanks to this fine gentleman above, who is also my favourite dish. "Schwa" involves more than just vowels, if you get a phonology book or something in detail at hand. Yes, I'm not sure if it's appropriate to call it "lenition" (+ "liasion") in this case. You think about what happens once the vowel fades, to a ”pro/pre". The "r" stands out. So normally an "r" in a position like this would be pronounced as in "read", "rise", yes? But now that the vowel's gone, the "r" plays a role similar to "doctor", "loner", ie what we call the "rhotic" thing, which most significantly happens the way around --- I weaR it vs I'm wea-Ring it. I mean just roughly explained like that, not in strict phonology, which is even more complicated. Savy?

问题是重音在后面，所以r不可能读的太重，读熟练了就好了。又如precious重音在前面，r 读出了就稍微明显一些。

Oh I mean "per -s-I-s" is wrong, but what you're getting at indeed sounds like "per" but actually only "pr" with no that "e" in between, and then the real question is about how you pronounce "pr" (pr - s-I-s). It's tricky.