Such as I [Sakyamuni Buddha] have spoken,
[on the clear instructions for purity]
is named [all] Buddhas’ speech [teachings].
Not such as spoken [taught],
is Papiyan’s [Mara: the evil demon king] speech.
– 楞严经 (释迦牟尼佛)
Surangama Sutra (Sakyamuni Buddha)
‘The Dharma’ (or ‘Buddhadharma’) refers to ‘法’ (or ‘佛法’) in Chinese, which means ‘teachings’ (or ‘teachings of the Buddha’), as formally represented scripturally in the sutras, which are later recorded discourses of the Buddha. The Dharma forms the core of the Triple Gem [of ‘all Buddhas’ (who have fully realised and teach the Dharma), ‘the Dharma’ itself, and ‘the Sangha’ (which is the community of all who strive to fully realise the Dharma, if yet to)]. The Dharma is thus what all Buddhists see as the centre of the Threefold Refuge.
‘Dharmas’, however, beginning with a small letter ‘d’ and ending with the plural letter ‘s’, such as ‘dharmas’ (if used after the beginning of a sentence), refers to ‘all physical and mental phenomena (of mind and matter; or “things”) in the universe’. It is also called ‘法’, but it is not ‘佛法’, as ‘dharmas’ is not about the Buddha’s teachings specifically, but all things in general, which includes both pure and defiled phenomena. Sometimes, the term ‘诸法’, which means ‘all dharmas’, is used to this effect. On a related note, ‘the universe’ is referred to as ‘the realm of dharmas’ or ‘the dharma realm’ (法界), where all phenomena of mind and matter occurs in.
The word ‘Dharma’ by itself means ‘truth’, which refers to ‘nature’ or ‘the way phenomena naturally is’. This is why Buddhists use ‘the Dharma’, to refer to ‘the truthful teachings of the Buddha, that awaken us to the truth of all dharmas (phenomena)’. (Thus, ‘the Dharma’ is ‘the path to truth’ too.) There are many dharmas, as various aspects of phenomena, but ‘the Dharma’ is a singular collective term for all of the Buddha’s teachings on the whole. (‘Dharma’ is in Sanskrit, while ‘Dhamma’ is the Pali version. Due to the above nuances of the word, it is often left untranslated.)
The saying ‘法无定法’, which means  ‘dharmas have no fixed dharmas’, or  ‘the Dharma has no fixed Dharma [method of expression]’ refers to two truths –  All phenomena (dharmas), being subject to change (are impermanent), and without enduring self-nature (are non-self). Thus,  All teachings of the Buddha (Buddhadharma), to skilfully awaken us to the above, have no fixed format as means. Yet, it is NOT that ‘anything goes’, as  the true nature of all phenomena (being impermanent and non-self) does not change, which means  the teachings leading to realisation of the above does not change in essence too. It is the realisation of this that leads to liberation, the transcendence of all defiled phenomena. If ‘anything goes’, the Buddhas would not see the need to teach us anything at all – not even guidelines for differentiating good from evil, what more the great path to purity!
As Mara’s followers believe ‘anything goes’,
they even forgo the clear instructions for purity,
that the Buddha taught as crucial foundations
for advancing towards Buddhahood.
The Buddha’s Admonition Against Making False Spiritual Claims