Your third sentence is most accurate. 1) It's first time for the baby to separate with his mom. -- the use of "with" in this sentence implies that the baby and the mom are staying together, which contradicts the idea that they are separating. You could use "with" if the mom and baby were separating from someone else. You must also use the article 'the' between 'it's' and 'first.' 2) It's first time for the baby to separate his mom. -- in this sentence, you did not use a preposition between 'separate' and 'his.' That changes the sentence to mean that the baby is physically separating his mom (almost like into pieces). You could say "the mom separated the two children,' but you cannot use it in the sentence above. You must also use the article 'the' between 'it's' and 'first.' 3) It's first time for the baby to separate from his mom. -- This is the best construction, as you are suggesting that the baby will be leaving his mom. Be sure to also use the article 'the' between 'it's' and 'first.' Thanks!

None of those sentences is quite right. A more natural way to say this is might be "It's the first time that the baby has been apart from his mom." Or, if you need to use "separate", it would be best to use a passive - "It's the first time the baby has been separated from his mom."

Hi Chemmie, We would like to convey the meaning that the baby will be away from his mom. The preposition that we should be using is "from", which is used to convey a meaning of separation through space or time. For example: Los Angeles is a five-hour drive FROM San Francisco. (Separation through space) Please isolate the patient FROM the other patients in the ward. (Separation through space) This operation will take place from 9 a.m to 3 p.m. (Separation through time) Therefore, your third sentence is correct. Since the baby is being separated from his mom, we should write: It's the first time that the baby is separated from his mom. *If you want to use "with", we can rewrite sentence 1 as: "The baby is made to part with his mom for the first time." In this case, "with" conveys the meaning of relationship or an association. Example: I love tea with cookies. Hope this helps. Cheers, Lance