How do loop through a list and take out several different variables?

I'd recommend generating a new list of each set of tweets using set comprehensions which are like list comprehensions. Then you can take the difference of the original list of tweets with the union of the sets of happy and sad tweets. Like so:

happy_tweets = {t for t in tweets for w in happy_words if w in t}
sad_tweets = {t for t in tweets for w in sad_words if w in t}
neutral_tweets = set(tweets) - (happy_tweets | sad_tweets)
print(list(happy_tweets))
print(list(sad_tweets))
print(list(neutral_tweets))

Which gives:

['Apple announces the release of the new iPhone 12. Fans are excited.', "I'm really excited to learn Python with @JovianML #zerotopandas", 'Spent my day with family!! #happy', 'Freecodecamp has great coding tutorials. #skillup', 'Wow, what a great day today!! #sunshine', 'This is a really nice song. #linkinpark']
['I feel sad about the things going on around us. #covid19', 'Why do bad things happen to me?']
['Check out my blog post on common string operations in Python. #zerotopandas', 'The python programming language is useful for data science']

The reason for using sets here, in lieu of just lists in happy_tweets and sad_tweets is to prevent duplicates in the case of multiple words from the lists matching the tweet string. A slightly more efficient method, though more verbose, would be to move these out into actual for statements, and break when the first word matches. But that is not a significant change to the time complexity.

As a bit of an aside, the reason the union operation must be grouped is based on operator precedence

Here is my code, maybe longer but I'm just trying to show you the solution in another way. Filter out all the happy and sad tweets, then every item left is the neutral one. Counting the number of tweets by their kind using len() func

tweets = [
    "Wow, what a great day today!! #sunshine",
    "I feel sad about the things going on around us. #covid19",
    "I'm really excited to learn Python with @JovianML #zerotopandas",
    "This is a really nice song. #linkinpark",
    "The python programming language is useful for data science",
    "Why do bad things happen to me?",
    "Apple announces the release of the new iPhone 12. Fans are excited.",
    "Spent my day with family!! #happy",
    "Check out my blog post on common string operations in Python. #zerotopandas",
    "Freecodecamp has great coding tutorials. #skillup"
]

happy_words = ['great', 'excited', 'happy', 'nice', 'wonderful', 'amazing', 'good', 'best']
sad_words = ['sad', 'bad', 'tragic', 'unhappy', 'worst']

happy_tweets_list = []
sad_tweets_list = []
neutral_tweets_list = [item for item in tweets]


for tweet in tweets:
    for word in happy_words:
        if word in tweet:
            happy_tweets_list.append(tweet)
            neutral_tweets_list.remove(tweet)
    for word in sad_words:
        if word in tweet:
            sad_tweets_list.append(tweet)
            neutral_tweets_list.remove(tweet)

happy_tweets = len(happy_tweets_list)
sad_tweets = len(sad_tweets_list)
neutral_tweets = len(neutral_tweets_list)

Another way of finding if substrings are in a string list.

tweets = [
    "Wow, what a great day today!! #sunshine",
    "I feel sad about the things going on around us. #covid19",
    "I'm really excited to learn Python with @JovianML #zerotopandas",
    "This is a really nice song. #linkinpark",
    "The python programming language is useful for data science",
    "Why do bad things happen to me?",
    "Apple announces the release of the new iPhone 12. Fans are excited.",
    "Spent my day with family!! #happy",
    "Check out my blog post on common string operations in Python. #zerotopandas",
    "Freecodecamp has great coding tutorials. #skillup"
]

happy_words = ['great', 'excited', 'happy', 'nice', 'wonderful', 'amazing', 'good', 'best']
sad_words = ['sad', 'bad', 'tragic', 'unhappy', 'worst']

happy_tweets = 0
sad_tweets = 0
neutral_tweets = 0


for tweet in tweets:
    if any(happy_word in tweet for happy_word in happy_words):
        happy_tweets += 1
    elif any(sad_word in tweet for sad_word in sad_words):
        sad_tweets += 1
    else:
        neutral_tweets += 1

print(happy_tweets)
print(sad_tweets)
print(neutral_tweets)

Output

6
2
2