Perhaps you might be interested in using \obeylines:
\documentclass{article}
\setlength{\parindent}{0pt}
\begin{document}
\begingroup\obeylines
\textbf{Graph Theory}
\textbf{\S1. Definitons and Miscellany}
Defns 1.0: Graph, vert, edge, adj, $E_n$, $K_n$, $P_n$,~$C_n$, sbgraph, induced graph, connected,
components, forest, tree
Thm 1.1: two equivalent characterisations of trees
\ldots
\textbf{\S2. Connectivity and Matching}
Defns 2.1.0: matching, independent, 1-factor
Thm 2.1: (Hall's Marriage Theorem) \quad Cor 2.2: (Defect Form)
Cor 2.3: (Polyandrous version)
\endgroup
\bigskip
Here
is
some
text
\end{document}
Using \obeylines allow you to drop the use of \\ for line-breaking, while wrapping occurs naturally. Also, scoping it inside \begingroup...\endgroup localizes the effect, as you can see in the end of the minimal example, where regular line-breaking is re-instated as spaces.
To maintain an alignment similar to tabbing, you could use some box-manipulation through \phantom and overlaps:
\begingroup\obeylines\raggedright
...
\textbf{\S2. Connectivity and Matching}
Defns 2.1.0: matching, independent, 1-factor
\mbox{}\rlap{Thm 2.1: (Hall's Marriage Theorem)}%
\phantom{Defns 1.0: Graph, vert, edge, adj, $E_n$, $K_n$, $P_n$,~}Cor 2.2: (Defect Form)
Cor 2.3: (Polyandrous version)
...
\endgroup
Note that the alignment is only achieved when there is no full line in the alignment of the "parent", otherwise the inter-word stretch might affect it. As an immediate resolution, I've used \raggedright.
\mbox "initiates" a paragraph, while \rlap inserts a zero-width box that is left-aligned/justified. This causes the so-called right overlap. Then, \phantom{<stuff>} sets <stuff> as a blank box so your alignment starts at the right spot again, immediately in line (horizontally) with $C_n$.
Best Answer
\+"causes the left margin of subsequent lines to be indented one tab stop to the right" (Lamport, LaTeX: A document preparation system, p. 202), so your second "Study of Biology" entry is placed at the third tab stop -- which doesn't exist, which in turn throws an error. The error may be avoided by adding\-(the complement to\+) plus a line break after the first "Blablabla".Another option is to do without
\+and\-and simply add\>at the start of lines 2 and 4.