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The words are almost identical, except for the extra ל. Try giving more context to your question. Where are you seeing these words and what about them are you trying to understand? Without any context, the two words mean "when they come" and "for when they come". Are those synonyms?
Like any library, Mi Yodeya offers tons of great information, but does not offer personalized, professional advice, and does not take the place of seeking such advice from your rabbi.