def pad_sentences(sentences, padding_word="<PAD/>"):
    """
    Pads all sentences to the same length. The length is defined by the longest sentence.
    Returns padded sentences.
    """
    sequence_length = max(len(x) for x in sentences) + 1
    padded_sentences = []
    for i in range(len(sentences)):
        sentence = sentences[i]
        num_padding = sequence_length - len(sentence)
        new_sentence = sentence + [padding_word] * num_padding
        padded_sentences.append(new_sentence)
    return padded_sentences