import re


def tag_hashtags(tweet):
    """
    Marks hashtags in tweet.
    INPUT:
        tweet: original tweet as a string
    OUTPUT:
        tweet with hashtags marked
    """
    words = re.split(r'\s+', tweet)
    new_words = []

    for word in words:
        if word and word[0]=='#':
            new_words.append('<hashtag>')
        new_words.append(word)

    return ' '.join(new_words)