DESCARTES : SUBSTANCE, MODE AND INNATE IDEAS
Descartes recognises infinite substance, finite substance, and modes. We have ideas of these things, or Descartes does. I take it that you want to know what the distinctions are between the three; which are innate and what role innateness fulfils in our knowledge of mind and matter.
I quote from Descartes to get us started. I believe in letting a philosopher speak for him- or herself unless there are compelling reasons against, and I don't see any here.
▻ SUBSTANCE : INFINITE AND FINITE
By substance we can understand nothing other than a thing which exists in such a way as to depend on no other thing for its existence. And there is only one substance which can be understood to depend on no other thing whatsoever, namely God. In the case of all other substances, we perceive that they can exist only with the help of God's concurrence. Hence the term 'substance' does not apply univocally, as they say in the Schools, to God and to other things; that is, there is no distinctly intelligible meaning of the term which is common to God and his creatures. 'Principles of Philosophy', I §51, The Philosophical Writings of Descartes, tr. J. Cottingham and others, I, Cambridge: CUP, 1985, 210.
'Infinite substance' is God, the only 'thing which exists in such a way as to depend on no other thing for its existence'.
'Finite substances' - these 'need only the ordinary concurrence of God in order to exist'. In this sense minds and material bodies are finite substances. Strictly only God is a substance at all but if we mark the point by describing God as infinite substance, we can a finite substance status to minds and material bodies. They are not independent of everything else full stop, but they are independent of anything but God.
What, then, of 'modes' ? This is a term we associate particularly with Spinoza but we should set Spinoza aside and consider just what Descartes says. At 'Principles of Philosophy', I §56 (Cottingham, I, 211) he defines 'mode' as 'what is elsewhere meant by an attribute or quality'. But he makes a distinction. He in fact reserves 'attributes' to denote general and unchanging characteristics of substances. They define what kind of substance something is. Modes, by contrast, are 'accidents' (from the Aristotelian to sumbebekos), features that substances merely happen to have. For instance, a material bodies have the general and (while they last) unchanging characteristic of being extended. Extension is an attribute of a material body. An accidental feature of such a body would be, say, its being or having been painted blue.
For more on the distinction between attributes and modes : 'Comments on a Certain Broadsheet' (Cottingham, I, 297).
At this point we need to step sideways to the cogito (‘I am thinking, therefore I exist’). Though implicit in the argument of the Meditations, the cogito does not appear in this form in any of them : Descartes says simply in Med. II that ‘I am thinking, I exist’ is necessarily true whenever he states or thinks it. (Cottingham, II, 17). For the ‘therefore’ we have to go to Med. IV (Cottingham, I, 127) and, for cogito ergo sum to Principles of Philosophy, I, §7 (Cottingham, I, 195) and the 1644 Latin translation of the Discourse.
Now, what strikes Descartes about the cogito is that its cogency rests on its being a ‘clear and distinct idea’. It is not obvious what Descartes means by a clear and distinct idea. The only place where he deals formally with the matter is at Principles of Philosophy, I, §§ 46-6 (Cottingham, I, 207-8). Roughly, however, we can say that an idea is clear if it is self-evident and distinct if it contains nothing but what is self-evident. And if an idea is clear and distinct we can rely on its truth. (Subject to a qualification to be mentioned below*.)
Movement can be faster now. Descartes supposes that he has a clear and distinct idea of God (Med. III; Cottingham, II, 32) as a perfect being : a being that necessarily exists and has all perfections including omnipotence, omniscience, benevolence and goodness.
But how can he have come by this idea of God ? Descartes turns to the consideration of ideas and classifies them (Med. III; Cottingham, II, 26) : ideas can be innate or present from birth (ideae innatae), they can be due to external sources such as perception or testimony (adventititiae), or they can be fictitious or invented (a me ipso factae).
Descartes satisfies himself that the imperfect (humans such as ourselves) can only have the idea of a perfect being if it originates from that being. Only a perfect being can cause us to have the idea of a perfect being. So the idea of God is innate; it cannot be adventitious or fictitious. But Descartes runs into trouble. He assumes the necessary truth of clear and distinct ideas to prove that God exists, since he has a clear and distinct idea of God as existing; and then appeals to the perfect nature of God, who as perfect is no deceiver, to guarantee the truth of clear and distinct ideas. This is (crudely stated) the infamous ‘Cartesian circle’. (It is also the qualification referred to above. Clear and distinct ideas are only reliable because God guarantees their truth. Even the cogito is not self-standing; without God to underpin its truth as a clear and distinct idea it would not be a dependable truth.*)
▻ KNOWLEDGE OF MIND AND MATTER
If we submerge ourselves in the topic of the Cartesian circle we will never resurface; there is not the space to consider it. But we do not need to appraise it; we only need to see what use Descartes makes of the existence of a perfect God of whom or which he has a clear and distinct idea.
Descartes believes that his mind is a continuant (a finite substance). He believes that the mind which enacts the cogito does not have merely a point-instant existence but exists over time. He also believes that he has a physical body (another finite substance) which, though distinct from his mind, is in some relationship with his mind; and that this physical body is also a continuant. And he believes that there are other finite substances – other minds, other bodies, and an external world of finite substances such as trees and mountains, stars and planets and so on.
What grounds these beliefs ? They are based on the assumption of the existence of a perfect God (infinite substance) who, as veracious, is no deceiver; it would be deceptive of God to allow Descartes to believe so naturally in the existence of his mind and body as continuants, in the existence of other minds and bodies, and in the existence of the external world, if none of these things existed. He makes this point in Med. VI (Cottingham, II, 62) exactly at the end of the Meditations.