Can I silently change the initial capital letter in a quotation to fit the quotation syntactically into my sentence?

No. As the MLA Handbook advises, “Unless indicated in square brackets or parentheses, changes must not be made in the spelling, capitalization, or interior punctuation of the source” (75). Let’s say your original source reads as follows:

Poets are the unacknowledged legislators of the world.

If you need to lowercase the initial letter of the first word to fit the quotation syntactically into your sentence, indicate the change in brackets:

In “A Defence of Poetry,” Shelley argues that “[p]oets are the unacknowledged legislators of the world.”

Work Cited

Shelley, Percy Bysshe. “A Defence of Poetry.” Poetry Foundation, 13 Oct. 2009, www.poetryfoundation.org/articles/69388/a-defence-of-poetry.

Although changing capitalization silently may be permissible—or even preferred—in other styles, in MLA style it is important to preserve the integrity of the scholarly record and present any quotations accurately. The style was developed by a field that has historically dealt with textual analysis. So even when using MLA style to write about a nonhumanities field or to create a work not engaged in textual analysis, follow the principles of MLA style and use brackets.

Work Cited

MLA Handbook. 8th ed., Modern Language Association of America, 2016.