You can use Alt + Enter keys to start a new line in a cell easily.

Double click into the cell, put the cursor at the place where you will start a new line, and then press the Alt + Enter keys together. And then you will see a new line is inserted into the cell.

You can also apply a formula to start new lines in a cell.

For example, the cell content is “ABCD”, you can change the content to below formula, and press the Enter key. Then enable warping text with clicking Home > Wrap Text.

="A"&CHAR(10)&"B"&CHAR(10)&"C"&CHAR(10)&"D"

If you want to start new lines to enter adjacent cell contents, you can use the clipboard to get it done easily in Excel.

1. Enable the clipboard with clicking the anchor at the bottom-right corner of the Clipboard group on the Home tab.

2. Select the rows you will add as new lines in a cell, and press Ctrl + C keys together to copy it. Then the range is added into the clipboard.

3. Double click into a cell, and click the copied range in the clipboard.

Then the copied range is added into this cell, and each row is added as a new line. See screenshot:

If you have Kutools for Excel installed, you can apply its cool Combine feature to add cell contents in the same row or column as a new line in cells easily.

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1. Select the range as you need, and click Kutools > Merge & Split > Combine Rows, Columns and Cells without losing Data.

2. In the Combine Columns or Rows dialog, please check the Combine Rows and New Line options, and click the Ok button. See screenshot:

Now you will see cell contents in each column are add as new lines into the first cell of corresponding column. See screenshot:

If check the Combine Columns and New Line options, and click the Ok button in the Combine Columns or Rows dialog, you will see cell contents in each row are inserted as new lines in to the first cell of corresponding row. See screenshot: