The different usages of -ため

Question

Roughly, my textbook taught me two major usages of -ため:

First, in context of a final clause, it is the colloquial equivalent to -ように and marks that the verbal expression connected to ために is the "goal" one tries to achieve through the verbal expression followed by the -ために phrase. Example:

他日泳げるために、毎日練習しています。

The second one is ため in context of causal clauses. For example:

大雪が降ったために、空港が使えなくなりました。

Especially in this case, except for paying attention to context and the general sense of the sentence, are there any other "hard" criteria by which I can distinguish ため being used in a causal function and ため being used in a final function? For example, when ため was used in a final function, I think the preceding verb was always in present tense. In the case above, the preceding verb is clearly set into past tense.

Answer 1

(Fist of all, what is 他日? And 泳げるために only translates into "because you can swim", not "in order to be able to swim", which is 泳げるようになるために.)

The combination with perfect aspect, adjectives or potential verbs only represents causal relationship ("because ..."). That's one.

• 美しいため: because it's beautiful
• 美しくあるため: in order to maintain their beauty (note; it's not impossible to interpret this as "because they remain beautiful")
• 飲んだために運転できないことも: you may be unable to drive because of drinking

As you say, when it represents a goal, the preceding verb must be irrealis form / imperfective aspect (what you call present tense) regardless of tense.

• 運転するため、飲まなかった: I didn't drink in order to drive (note; it's not impossible to interpret this as "because I was scheduled to drive")

Answer 2

Your consideration is very interesting, but there are exceptions like this:

大雪が降ってきたため、空港が使えなくなりそうだ。
Since it came on to snow hard, it seems that we won't be able to use the airport.

In this case, なりそうだ is not past tense but future one. Then, how do we distinguish these two usage? I can't think of any ideas about distinction. Maybe there are no definite criteria.