The {lo'ai ... sa'ai ... le'ai} replacement construct is used to erase some recently uttered string of words and replace them with something else. It is comparable to the "s/foo/bar/" expression commonly used on IRC.
By saying {lo'ai srera sa'ai drani le'ai}, you are asking the listener to erase the most recent occurrence of {srera} in your text stream, replace it with {drani}, and reparse. For example, {lo'ai cakla sa'ai ckakla le'ai} can be translated as, "I didn't mean {cakla}, but {ckakla}."
It's possible to switch the order of the text to be replaced and the replacement by saying {sa'ai drani lo'ai srera le'ai}.
It's also possible to omit either the text to be replaced or the replacement, leaving it up to context. For example, {sa'ai ckakla le'ai} means, "I meant {ckakla}," and leaves it up to context what string of words should be replaced by {ckakla} (in this case, probably the word {cakla}). Similarly, {lo'ai cakla le'ai} means, "I didn't mean {cakla} (but something else)."
It is interesting to note that {lo'ai ... le'ai} can be thought of as an "error quote," which is the traditional (arguably poor) gloss for {lo'u} and {le'u}. In other words, {lo'ai} and {le'ai} are the real "error quote" cmavo.
Naturally, one can erase a specific string of words recently uttered by saying {lo'ai broda brode .ua .ue .ui sa'ai le'ai} — essentially replacing {broda brode .ua .ue .ui} with the empty string.
The converse case is somewhat special: {lo'ai sa'ai se setca le'ai}, rather than replacing the most recent occurrence of the empty string (which would be useless), actually tells the listener to insert the string {se setca} at the appropriate place in the recent text stream. Where the string is to be inserted is left up to context.
By saying just {le'ai}, omitting both the text to be replaced and the replacement, one can acknowledge the fact that one has made an error while leaving it completely up to context what the actual error was. Thus, {.i .ai mi cakla ... le'ai} means roughly, "I'm going to be chocolate. ...err, you know what I mean."
To summarize, the different forms of {le'ai} expressions are as follows:
— Daniel Brockman