Yes, could is conventionally the past tense of can, but as a modal it also handles present meanings, so the label tells only part of the story.
Say He could speak five languages when he was young about my uncle, and the past label fits. Yet Could you help me? refers to now, so the difference there is politeness, not time.
Take one person: my uncle speaks five languages today, so you say He can speak five languages. The verb can points to the present, because the ability is true right now.
Years earlier, the same sentence about him needs could: He could speak five languages when he was young. You shift can to could because the ability belongs to the past, so the conventional past form fits.
Notice the cause and result here. The time changes, so the form changes. Can keeps the ability alive in the present; could shows the ability held then, which also suggests it may not hold now, or maybe the speaker is just not sure.
Now keep the very same uncle, but change what you want. You need a favor from him today, so you say Uncle, could you help me move this box? He is standing right there, and you are talking about right now.
This use of could is not past at all. It is a polite request or a way of asking permission, so the same word that pointed backward a moment ago now works in the present. The distance it adds is social, not time-based. The past-tense label has its own limit, taken up below.
Where the past-tense label stops working
The past-tense label has one important limit. For a single completed success in the past, English usually prefers was able to over could in affirmative sentences, so the conventional past form does not cover every past use.
Take the uncle again. Suppose he opened a stuck jar at one family dinner. People naturally say He was able to open the jar, because the sentence reports one specific achievement, not an ability he had in general.
This is about naturalness, not strict grammar. He could open the jar sounds less natural in the affirmative, yet the negative behaves differently: He couldn't find his keys is completely normal.
So the limit applies to affirmative single-event sentences. When the verb is negated, couldn't works fine even for one occasion, which makes the pattern easier to feel than to explain.