こんにちはは えいごで "Hello"です / "Konnichiwa" is "Hello" in English.

こんにちは

this is a greeting expression which means "Hello.".

こんにちはは　"Hello"です。

this means "'Konnichiwa' is 'Hello'".

こんにちはは　えいごで　"Hello"です。

I put "えいごで" which means "by English (in English)".  This means "'Konnichiwa' is 'Hello' in English.".

"Hello"は　にほんごで　なんですか？

You can also ask "What is 'Hello' in Japanese?" with this sentence. If you don't know the Japanese word for the English word that you know, you can ask Japanese friend using this phrase.

にほんごで メールを かきます / I write mail in Japanese.

にほんご

means "Japanese language.".

にほんごで

"で" is a method marker. So, it means "by Japanese" which is "in Japanese" in proper English. You can use "で" to express what language or character type you use to do the action.

にほんごで　メールを　かきます

"めーる/ mail". "かきます/ write". This means "I write mail in Japanese.".

えいごで　はなします

"えいご/English", "はなします/speak, talk". This means "I speak in Enlish.".

なにで たべますか？ / By what do you eat?

で

this is a method marker.

なにで

なに means "what". なにで means "by what".

なにで　たべますか？

"たべますか? / eat?". So, this means "By what do you eat?". This is used when you want to ask which the person eat by chopsticks, spoon, fork or etc...

はしで　たべます。

Answering to the above question with "I eat by chopsticks.". "はし / chopsticks.".

わたしは ぱそこんで えいがを みます / I watch a movie on PC.

ぱそこん

this means "PC(Personal computer)".

えいが

this means "movie".

みます

this means "see, watch, look". This is masu-form.

ぱそこんで

the particle "で" means "by". It is the method marker in this case. This means "by PC".

わたしは　ぱそこんで　えいがを　みます

So, this means "I watch a movie by PC.".

いきましょう / will go

いきます

this is the masu-form of "go".

いき＋ましょう

Keep the masu-form stem "いき", and change the ending to "ましょう".

いきましょう

this means "go" with willingly accepting invitation by other.

いっしょに　かふぇに　いきませんか？

This means "Shall we go to cafe together?". And you are asked this, and you accept this invitation and want to say "will go!" in a way in which your attitude is like "Yes! let's go. I agree.". So, you say...

いきましょう

いっしょに かふぇに いきませんか？ / Shall we go to a café together?

いっしょに

It means "together".

かふぇ

It means "café".

いきません

this means "don't go".

いきませんか？

this is a question form because "か" is put at end. This means "don't go?, shall we go?" with which you can ask someone to go. The reason why this is negative form as "don't go?" is, that it is easier for the listener to answer no to this question. You usually ask in this way.

いきますか？

this is in a positive form. this is also correct. It means "go?, do you go?". It's more expecting the positive answer from the listener. However, the above negative way is used more often.

いっしょに　かふぇに　いきませんか？

It means "Shall we go to the café together?".

どこで わいんを かいますか？/ where do you buy wine?

わいんを　かいますか？

"わいん/wind", "かいます/buy". This means "You buy wine?".

どこ

means "where". 

どこで

put the particle "で" at the end. It means "at where, in where".

どこで　わいんを　かいますか？

Means "Where do you buy wine?".