4.026 The meanings of simple signs (words) must be explained to us for us to understand them.
We make ourselves understood, though, with sentences.
Black (p. 172) offers this rather free paraphrase: “The meanings of words have to be explained for us, have to be shown, but once this has been done, we use propositions to make ourselves understood, to communicate thoughts.” It seems to me, though, that the meaning might be more that, while words need to be explained, the explanations offered can only take the form of sentences. Meaning and understanding take place in the medium of sentences, not words (even though, in a sense, sentences are made up of words). Sentences consist of words in the way that the human body consists of such things as carbon and water. You can break the one down into the other, but you cannot produce the greater simply by adding or mixing the ingredients.
Compare with 6.54. Wittgenstein there seems to contrast understanding a person's sentences with understanding the person, so that we can understand him yet see that his sentences are nonsensical. According to 4.026, though, we could only understand him by means of his sentences. If these sentences are nonsensical, it is obviously difficult to see how we could understand him by means of them.
My initial reaction: You need to learn the meanings of words, but once you know them you can understand new combinations of them in sentences. The sensible components of language are arbitrary and so must be learned, but the senses (meanings0 expressed by means of them are, what, already intelligible? This would be very different from the later Wittgenstein, perhaps even the Wittgenstein who comes later at 5.62, for instance. If “senses” are “intelligibilities” then this is uncontroversially true, I suppose, but hardly as informative as it sounds.