4.024 To understand a proposition means to know what is the case if it is true.
(One can therefore understand it without knowing whether it is true.)
One understands it if one understands its constituent parts.
The first two sentences here seem straightforward enough. The last is problematic, because you might think you do not understand “The cow milks the boy” even though you understand the individual words. But knowing what milking (a cow) is does not tell us what is meant here. So we do not understand the constituent parts of this sentence. We understand parts of other sentences that look the same, that is all.