Nicole: What can I do to live a long life?
CORRECT!

Nicole:I thought about this.
Correct: I've been thinking about this.
EXPLANATION:
We use 'have been thinking' as a form of PRESENT PERFECT CONTINUOUS OR PROGRESSIVE to show that something started in the past and has continued up until now.

Nicole: First, you should exercise.
Correct: First, I should exercise.
EXPLANATION:

Since you used the pronoun 'I', then you must observe its consistency in your sentences.

Nicole: Second, you should have good habits
Correct: Second, I should have good habits.
EXPLANATION:

Since you used the pronoun 'I', then you must observe its consistency in your sentences.

Nicole:Third, you should have a good relationship with others.
Correct: Third, I should have a good relationship with others.
EXPLANATION:

Since you used the pronoun 'I', then you must observe its consistency in your sentences.

Nicole:Lastly, you should smile every time you can.
Correct: Lastly, I  should smile every time.

Nicole:I think this is the best way to live a long life.
Correct: I think these are the best ways to live a long life.
EXPLANATIONS: 

    We use 'these' when referring to many things. It is the plural form of 'this.

  Singular means just one of the person, animal or thing which the noun refers to and you will use "is". Plural means more than one and you will use "are".

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Hello, Nicole! Thank you for doing your homework. Kindy take note of the corrections that I have made for you. You did an amazing job, Nicole! See you on Thursday! Take care always^^

-Teacher Isabel