Pretty good job, Yi Zhang. 

PART 1:

Corrections in ALL-CAPS because I can’t bother with formatting. 

1. GOOD:

If birds see a hole, they fly into it. - theory on birds as a species behaviour of all.

2. GOOD:

If birds see a hole, they will fly into it. - prediction，focus on THE FACT THAT birds will fly into it.

3. PROPER STRUCTURE, WRONG INTERPRETATION:

If birds saw a hole, they would fly into it - THEY DON’T SEE ANY HOLE, SO THEY DON’T FLY IN.

4. SAME COMMENT AS FOR EVA: Your version is often used in colloquial language, although the if-clause should really be back-shifted for a proper 3rd conditional:

If birdS HAD SEEN a hole, they would have flown into it. - Birds did not see the hole，（therefore？）they did fly into the hole. I do not think I understand this. I BELIEVE YOU UNDERSTAND. THE WORD “THEREFORE” WOULD EMPHASIZE THE CAUSAL RELATIONSHIP.

5. A MIXED CONDITIONAL. This works if we take “saved” as an an adjective, or we could use “safe.”

If (THE) birds had flown into a hole, they would be saved. - (THE) birds all DIED, but they could (HAVE) beEN saved. 

Another option would be the 3rd conditional for both clauses:

If (THE) birds had flown into a hole, they would HAVE beEn saved.

Hi Phil. I can fully understand the answer of part1. As for the answer of part two. I am still confused.I think the question is a little beyond me. It can wait, I will check your answer maybe few months layer, and I hope to understand it then.

You helped me a lot today Phil. Thank you very much.

Part 2

Very Advanced:

Other than putting the main verb in the subjunctive mood, “If I were called upon”, we can use an auxiliary verb such as “to be” or a modal auxiliary that is obviously subjunctive. This is more formal in style: “If I were to be called upon” / “If I should be called upon”. Note that the sentence with “if I should” seems more likely, while the one with “if I were to be…” seems very unlikely. For an even more literary style, we can omit "if" and use inversion in these cases: “Were I to be called upon” / “should I be called upon.” These are not questions; the inversion replaces the word “if”. This grammar goes back over a thousand years (it’s the same in High German).

Colloquial American:

In informal American English, you may hear “would” used in the if-clause. While this can be correct in some cases (when “would” has it’s original meaning of “to want” / “to be willing”), in most cases it’s considered an error. This grammar is, however, used a lot in High German, where it’s considered perfectly correct, especially for those verbs whose indicative and subjunctive forms are the same.

Part 1

Good question, Tang Lei. In the if-clause of a second conditional sentence, we have to use the subjunctive — theoretically. In reality, however, the subjunctive form is the same as the indicative form for all verbs except “to be”. With “to be”, the subjunctive is “were” for all grammatical persons (as opposed to “were” and “was” in the indicative mood). For this reason, today’s native speakers are not very sensitive to the subjunctive mood, and will often just use the indicative. UK speakers are especially apt to ignore the subjunctive. Ignoring it on a test of British English would usually not cost you any points. For an academic exam in American English, however, you should use the subjunctive according to the traditional rules. Many Americans use the subjunctive (present and past) correctly most (or all) of the time, some only in formal style, some not so much. Some use it correctly but have no idea why it’s correct :)

Hi Phil, you just gave me some answers of my posted question about sencond conditionals. And I have another question about that. I try to write my question here in order other people can read this. And your answer might help others who got the same question.

1. should we always apply the subjunctive mood in a second conditional ?

2.should we always use "were" instead of "was" in the subjunctive mood ?

Thank you in advance.