Ggtcf

Question: How can i push the second line in such a way that it will start exactly, where the first line started without disturbing ideal line spacing?

My MWE is:

documentclass[12pt, a4paper]article
usepackage[top=0.7 in,bottom=0.5 in,left=0.6 in,right=0.6 in]geometry
usepackage[utf8]inputenc
usepackageamsmath
usepackageamsfonts
usepackageamssymb
usepackageparskip
%line spacing
renewcommandbaselinestretch1.10 
%reduce top margin
addtolengthheadsep-0.45cm

begindocument
(1)~This is the first line. This is the first line. This is the first line. This is the first line. This is the first line. This is the first line.


enddocument 

To expand on the comment by @moewe: you can use the enumerate environment to number the lines, which aligns the text automatically. By default the numbers appear as 1., 2. etc. There are several ways to change the appearance of the numbers. An easy way is to use the enumitem package with the option shortlabels as in https://tex.stackexchange.com/a/2294/, and specify the label as [(1)] at the start of your enumerate environment. MWE:

documentclass[12pt, a4paper]article
usepackage[top=0.7 in,bottom=0.5 in,left=0.6 in,right=0.6 in]geometry
usepackage[utf8]inputenc
usepackageamsmath
usepackageamsfonts
usepackageamssymb
usepackageparskip
usepackage[shortlabels]enumitem
%line spacing
renewcommandbaselinestretch1.10 
%reduce top margin
addtolengthheadsep-0.45cm

begindocument
beginenumerate[(1)]
item This is the first line. This is the first line. This is the first line. This is the first line. This is the first line. This is the first line.
item This is the second line.
endenumerate

enddocument

Result:

Edit: if you don't want an indent then you can change the leftmargin parameter of the enumerate environment. If you set it to labelwidth then the margin will be just big enough to allow the number to be printed and the indent is gone. However, the width of the label is slightly bigger than the printed number, so it still looks a little bit misaligned with regular text. This may not necessarily be bad typographically, but if you want you can shift the number further left by manually setting the labelwidth.

MWE:

blindtext
beginenumerate[(1),leftmargin=labelwidth]
item This is the first line. This is the first line. This is the first line. This is the first line. This is the first line. This is the first line.
item This is the second line.
endenumerate
beginenumerate[(1),labelwidth=7.6mm,leftmargin=labelwidth]
item This is the first line. This is the first line. This is the first line. This is the first line. This is the first line. This is the first line.
item This is the second line.
endenumerate

Result:

In case that you need some more that a conventional list, or you like a very simplified syntax, there are linguex. Note that as show in the example below, it matter if there are more of one blank line between the item and a not numbered paragraph, unlike in most situations in LaTeX documents, where the number of blank lines (=par) is irrelevant.

documentclassbook
usepackagelinguex
usepackagelipsum % for nice dummy text (always "This is the first line" is boring ...)

begindocument

lipsum[1][1-2]

ex. lipsum[1][3-4]

ex. lipsum[1][5-6]

lipsum[6][1-3]

ex. lipsum[1][7-9]


lipsum[2][1-3]

ex. lipsum[3][1-3]

ex. lipsum[4][1-3]

lipsum[5][1-4]

enddocument