when to use te iru vs desu and other verb forms?

Toritoribe said:

1)
Although 勉強する is a durative verb (継続動詞), 僕は毎日勉強しています is a present habit, not present progressive, because of 毎日. The second one 僕は毎日勉強します is future, such like 僕は明日から毎日勉強します I will study everyday from tomorrow . 毎日勉強します can express the present habit when using with specific temporal adjectives such like 毎日夕食前に勉強します. This 勉強します actually refers to the whole action of studying of the day, i.e., it means that the speaker starts studying sometime before dinner and usually finishes it before dinner everyday.

2)
太い is applied to body parts (e.g. 首が太い, 腕が太い), but can't be used for the appearance of the whole body.

3)
ここ is the only information the speaker needs to tell in the former sentence. For instance, it's an answer of a question "Where is your grandfather?". The questioner and answerer(= the speaker) both know that they are talking about the present location of the grandfather, so います is not necessary to be mentioned. Furthermore, ここです does not always show the location of existence. In a context お父さんはいつも二階でタバコを吸います。おじいさんはここです。, ここです represents the location where the grandfather has always smoked. It's obvious that the speaker is talking about the location of smoking from the context, so it's OK just to use です instead of でタバコを吸います. おじいさんはここにいます can't be used here, of course.
As for the latter one, います is also necessary other than ここ. It's used in a situation, for instance;
「おじいさんはどこかに出かけていますか」「いいえ、（おじいさんは）ここにいます」
or
お父さんは二階で寝ていますが、おじいさんはここにいます,
thus, the questioner/the one the speaker is talking to doesn't know the information of いる, so the speaker needs to mention it.
It's not wrong to answer （おじいさんは）ここにいます to the question おじいさんはどこにいますか, though.

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