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With the interest of the unique function of “for” as a casual coordinating conjunction and its survival in English as its usage has decreased dramatically, the study of the frequency of conjunctions “for, as, and because,” as well as “for” as a preposition has been conducted. Previous works examine conjunctions in terms of grammaticalization, pragmaticalization, and subjectification based on their historical semantic development. This study introduces the use of the Corpus of Historical American English (COHA) and the Corpus of Contemporary American English (COCA) to analyze how the conjunctions in question have risen or declined and proposes the possible causes by examining the sources of data collection in the corpus. Based on the frequency in a corpus, “for” as a coordinating conjunction and “as” as a subordinating conjunction have fallen in its usage while “for” as a preposition and “because” have been used more. “For” has gone through the process of reanalysis (Harris & Campbell. 1995, Hopper & Traugott. 2003) and pragmaticalization (Traugott. 1999, Traugott & Dasher. 2002, Detges & Waltereit 2009). The sources of the collection show that the first two conjunctions “for” and “as” are found mainly in fiction and academia, but “because” is found most often in a spoken language. “For” as a preposition is found in many sources, so it does not show any implications. This gives an explanation to why the conjunctions “for” and “as” have declined in their usage. It is because they are mostly used in fiction and academia which account for only a small proportion of a language usage in general when compared to a spoken language. The limitation of this study is the mis-annotations of a preposition “for” as a coordinating conjunction in the corpus.