"He is becoming a child."
Translation:הוא הופך לילד.
You do realize that להפוך has not been introduced in any of the nine lessons that cover the unit "Verbs: Present - Pa'al", right?
It is supposed to be covered in Lesson 9 —the form הופכים is indeed mentioned in the overview— but it actually is not included in any of the sentences of that lesson.
We first encounter the verb להפוך in this very sentence, as part of the Strengthening (practice) session.
"sim = שים" (לשים = to put) is in Pa'al as well. The root is שימ and because of the 'י' in the middle its conjugation is a bit unusual.
In hifil it's השים, or להשים. It's mainly used nowadays for job placement (השמה) like Almog said, or for expressions like "מבלי משים" (=unintentionally).
In hufal it's indeed הושם, I still think it doesn't sound appropriate here, but I guess it's a question of style.. :-)
The infinitive is the basic form of the verb, the way you'd find it in a dictionary. In English it is usually preceded by the particle to. In French it would be e.g. parl
er, in Spanish habl
ar, in German sprech
en and in Hebrew
Compare "I need to talk to you" (to-Infinitive) or "Can you please talk to him for me?" (Bare Infinitive) with "Stop talking!“, “He talks in his sleep", "He always talked big“.
When, on a previous sentence, someone posted that DL marked "turn into" wrong, I looked up הלך in my 1965 Alkaly 2-vol dictionary. Along with "turn over" and other meanings, he had "turn into" which my father had underlined and added "become" in pencil. Lost Dad in 2005. But I see his pencilled notes in my Hebrew and Yiddish dictionaries every day. Your erudition reminds me of him.