How does Orwell's use of symbols in the novel reveal themes in 1984?

Most of the symbols represent the freedom of thought and choice to do what one wants when one wants--for example, the diary Winston purchases and writes in just out of the view of the telescreen, and then hides each time he leaves the apartment.  In it, he writes about how he hates Big Brother (another symbol--since he is not a true person...just a figure head of the society).

"The Bells of St. Clements" and the paperweight Winston purchases in the black market are also symbols of this freedom...a time in the past when people could do nothing or everything as they chose to.  Of course, the paperweight also symbolizes the love affair Winston has with Julia--it's transparent, beautiful, and fragile just like their relationship.  The coral center represents their love, but since it is easily seen, it also represents the fact that they have been observed the entire time of their trist even if they didn't realize it entirely.

The Prole Woman and her song is also a symbol of freedom and a time that has been which Winston hopes will come again.

There is also the Chestnut Tree Cafe. A place that Winston sees as an area to exchange ideas, but turns out to be more of a "holding cell" for the doomed.  The three conspirators drink and eat there until they finally disappear forever, and this is where Winston basically lives at the end of the book.

In The Giver, how did Jonas react to the children’s war game?

Jonas, unlike all the other children in the community, understands what war really is.  Since the other children have no conception of what death means, they are unable to truly grasp what war is.  Jonas knows because he has received memories of war from the Giver and realizes that war means death, and that death means horrible pain for the one dying. It means loneliness, sadness, and grief for those still living. 

Because of this, Jonas gets very upset when he finds the other children "playing at war". He gets choked up and tells the others to stop playing that game, that they don't know what they are doing. They respond by saying that it's just a game. Jonas knows better, though, and can't participate, so he leaves.

What does Shakespeare's use of euphemism in Macbeth's soliloquies show about Macbeth? Esp. in the soliloquy of Act 1, Scene 7

That is a very good answer!
I thought I might add an interesting point that merely conforms with what you have said as a sideline illustration.
In Act IV scene I the witches describe what they do as –a deed without a name-
In his soliloquies prior to Duncans murder Macbeth refers to the act in much the same way. He uses the word do rather than name the deed.
-If it were done, when tis done, then twere well / It were done quickly (I.vii.1-2)-
-I go and it is done (II.i.62)-
Done is a funny word in Shakespeare as it usually refers to sex, equalising the act of murder with consummation.
Elsewhere the deed is it… Macbeth speaks the forbidden word but once
-My thought, whose murther yet is still fantastical-
And this is equivocation, while admitting he is thinking of murder he also suggests that thought itself is murdered.
Throughout Macbeth dialogue dwells obsessively on the unnameable, both action and identity are lost in the confounded language of the play.

Read- Shakespeare: Violation and Identity, by Alexander Leggatt for more detail

How does Romeo and Juliet's marriage in Act 2, Scene 6 foreshadow future events?

The fact that the couple secretly marries without benefit of Juliet's father's permission hints to us that something will go wrong because the rules for marriage have been broken. Paris follows these rules, but Romeo breaks them. The Friar's agreement to marry them is a huge faux pas on his part as well. 

The impetuousity on the part of the Friar, Romeo, and Juliet to marry foreshadows the later rash decisions of Romeo and Juliet to immediately kill themselves without thinking it through. Even the Friar's rashness in marrying the couple foreshadows his act later on of making the potion for Juliet to get her out of the marriage to Paris.

Of course, the fact that Romeo and Juliet are the only children of feuding families foreshadows that this cannot end well. Tybalt became very angry with Romeo at the party, and we know that his hot temper won't leave this alone. The marriage further complicates this situation, foreshadowing the duel to come later between Tybalt and Romeo.

What games are being played in Chapter 5? (Hint-more than just the ones in the game room.)

As mentioned above, besides the games found in the game rooms, mind games are also played in this chapter. Various people play mind games on others in order to achieve a desired end. For instance, the mind game on Ender by the authorities still continues as he is moved to a new dormitory where he undergoes further isolation. In a discussion between Colonel Graff and an unnamed individual, it is evident that the plan is to ensure that Ender’s experiences foster independence and strength. Ender is so distraught that he cries in bed on the first night at his new dormitory. Ender also plays a mind game on Bernard and the rest of the boys. By devising a strategy that would strip Bernard of his popularity, Ender succeeded at making the rest warm up to him. He used a technology gimmick to embarrass Bernard for his bullying.

What memories does Jonas gets after those of war, and why does Jonas dislike the good guys/bad guys game now?

After having received the memory of war, Jonas doesn't want to play the goodguys/badguys game because he knows the pain the reality of war, the concept on which the game is based. The Giver gives Jonas tidbits of happy memories after the memory of war which include the beauty of art, the wonder of nature and a relationship with animals, solitude, and his favorite, families. Jonas learns he can find the names of his grandparents at the Hall of Open Records, and expriences warmth, happiness, and love at the holiday celebration (Chapter 16)

Twins are unacceptable is they are identical because of the confusion they might cause in the community, which values order above all things (Chapter 17).

Rosemary was the person who was supposed to be the Keeper of Memories before Jonas.  She was also the Giver's daughter.  During her training she was overwhelmed by the trauma of memories such as loneliness, loss, poverty, hunger,and terror, and asked for release. At her release those memories returned to the community, which itself was overwhelmed for a time, because the Giver was too distraught himself to help them through (Chapter 18). Recent and present day examples of release might be the killing of millions of Jews by the Nazis during World War II, and abortion of unborn children.

In the turtle and birds story, does the turtle represent the colonizers and the birds the Igbo?Am I correct to say that in the story about the...

According to the analysis available at the following link, you are correct about what the story represents.  Even the introduction of the tortoise is clearly indicating the European powers who heard of this feast in which they wanted to take part but weren't sure how to do it.  They saw Africa as a new place to get raw materials and cheap labor and so descended upon it just as the tortoise found a way to rise into the air.

The tortoise used his great skill with language to trick the birds into giving him all the best things at the feast, just so the colonists find ways to convince the natives that they are going to be helpful and not harm the native customs, etc.