You said "Rawm cannot be on a presented kasrah or dhammah put on a letter to get rid of the occurrence of two saakin letters juxtaposed." I dont understand this sentence well.

Question
Salaam Alaikum wa Rahmatullahi wa Baraktuhu.

I'm learning Stopping on the Ends of Words, in Lesson One:

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh,
Some terms are hard to put in English.  We used the term "presented" dhammah and kasrah to denote a kasrah or dhammah not part in the original make-up of the word.  There is a general rule in Arabic that you cannot have two saakin letters meeting between two words.  If this happens, then something must change so that there are not two saakin letters between two words.  The usual case is that the last letter of the first word which had a sukoon, acquires a vowel.  This vowel then is not due to congugation of the verb or noun, nor is a permanent vowel, instead it is a transitional vowel placed on the last letter of a word which had a sukoon so that two saakin letters do not meet in pronounciation.  An example of this is the  saakinah in the word: ; this is a fixed sukoon and the way this word is normally written.  If however, a saakinah letter is the first pronounced letter of the following word, then the on the end of  acquires a dhammah to avoid the occurence of two saakin letters between two words.  An example of this is in the two words: .  The in the word now has a dhammah.  This is what we mean by "presented" dhammh.  This is not an original or fixed dhammah nor a result of congugation, so is not allowed in this case, when stopping on the word . 
An example of a conditional or "presented"  kasrah is  the word in aayah 2 of surah Yunus.  The verb is a congugated one and the on the end has a sukoon due to the verb being a command form, but because the first pronounced letter of the next word is saakin, the acquires a kasrah: .  is not allowed on this word either because the kasrah is not a fixed one, but an acquired kasrah as result of the meeting of two saakin letters between two words. 
 
Insha' Allah this makes things clearer. 
Wa iyyaakum wa-l-muslimeen.