When you copy literal phrasings of another author (or your own, previous work), you're citing another text. Some quick guidelines:

• Always use quotation marks: "..."
• Refer to the source immediately after the citation
• If you use a footnote reference style, also add the page number
• Is it a citation of 40 words or more? Then it's a block quote. Block quotes start on a new line, are indented, and don't need quotation marks

There are some advantages to citing:

• You can cite others to support your own thesis
• You can focus on the way the author phrases their findings
• You can contrast what the author claims with what you claim
• Citing puts the emphasis on the researcher and not their findings

But there are downsides as well:

• It's harder to foreground your own voice
• It's not ideal for the flow of your text