To distinguish between sentences #3 and #4 ("I can do it" and "I can't do it"), we rely on the context and the emphasis on the word "can't."
In normal speech, the "t" sound in "can't" is often dropped, so we need to pay attention to other cues.
To distinguish between the two sentences:
1. We rely on the context: Pay attention to the words that follow the "can" statement. If the statement is followed by a positive action or possibility, like "do it" in the case of sentence #3, it suggests the capability to perform that action.
2. We listen for emphasis: Even though the "t" sound may not be pronounced, we can emphasize the word "can't" by emphasizing the "n" sound at the beginning. We might also pause slightly before saying the word "can't" to give it more emphasis.
In summary, context and emphasis are key in distinguishing between "I can do it" and "I can't do it" when the "t" sound in "can't" is dropped in normal speech.