Final answer:
To add to an existing file, you should open the file in append mode using the letter 'a'. When manipulating text, particularly for word changes or spelling corrections, close attention to instructions is crucial for accuracy.
Step-by-step explanation:
You have three modes for opening a file, and if you wish to add to an existing file, the mode you should use is "append". When opening a file in this mode, you typically use the letter 'a' in the line of code. This would look like open('filename.txt', 'a') in many programming languages, such as Python. The append mode allows you to write data to the end of the file without overwriting the existing content.
When working with files and making word changes, it's essential that you carefully follow instructions to ensure correct spelling and avoid errors in your code, just as when writing words ending in <tle> or <ttle>. Moreover, in English language spelling rules, the letter <c> spells the [s] sound when followed by <e>, <i>, or <y>, which must be kept in mind while implementing text processing features.