The 1841 British edition accidentally omitted the conjunction that after know. Normally in the Book of Mormon text, we expect the that in this kind of subordinate construction, although there are a few cases where the that is lacking, as nearby in Alma 10:9: “wherefore I know he is a holy man”. For each case of “to know (that) S”, where S stands for a sentence, the critical text will follow the earliest textual sources in determining whether the that is there or not. For some statistics regarding the occurrence of that after the verb know, see under 1 Nephi 1:3. Also see the general discussion under that in volume 3.
Summary: Restore in Alma 18:3 the subordinate conjunction that which was accidentally skipped by the 1841 typesetter.