Until you're more used to doing this sort thing, you should write out the original statement explicitly before you negate it. The definition of linear dependence is:
$S$ is linearly dependent iff there are finitely many vectors $x_1, \dotsc,x_n\in S$ (all distinct) and scalars $a_1,\dotsc,a_n \in F$ such that: the scalars are not all zero, and $\sum_{i=i}^n a_i x_i = 0$.
The negation of that, sticking to mathematical English, is:
$S$ is linearly independent iff for every set of finitely many vectors $x_1, \dotsc,x_n\in S$ (all distinct) and scalars $a_1,\dotsc,a_n \in F$: if the scalars are not all zero, then $\sum_{i=i}^n a_i x_i \ne 0$.
Note that "if the scalars are not all zero, then $\sum_{i=i}^n a_i x_i \ne 0$" is equivalent to its contrapositive: "if $\sum_{i=i}^n a_i x_i = 0$, then the scalars are all zero", which is the more common way of defining linear independence.
More formally, here's linear dependence:
$$\begin{equation}\begin{split}
(\exists x_1, \dotsc,x_n\in S)[&\bigwedge_{1\le i<j\le n} x_i\ne x_j \\
&\wedge (\exists a_1,\dotsc,a_n\in F)(&\bigvee_{1\le i\le n}a_i\ne 0) \\
&&\wedge \sum_{1\le i\le n}a_i x_i = 0 \\
]
\end{split}\end{equation}$$
The form of this statement is:
$$
(\exists \vec{x} \in S)[A(\bar x) \wedge (\exists\vec{a}\in F)(B(\vec{a})\wedge C(\vec{a}, \vec{x}))] \tag{*}
$$
where
- $A(\vec{x})$ is the big conjunction saying that the $x_i$ are all distinct,
- $B(\vec{a})$ is the big disjunction saying that some $a_i$ is not zero, and
- $C(\vec{a},\vec{x})$ says that the sum of the products $a_i x_i$ is zero.
Negating (*) gives:
$$
(\forall \vec{x} \in S)[A(\bar x) \to (\forall\vec{a}\in F)(B(\vec{a})\to \neg C(\vec{a}, \vec{x}))] \tag{$\neg$* 1}
$$
Rearranging that and using the contrapositive gives:
$$
(\forall \vec{x} \in S)[A(\bar x) \to (\forall\vec{a}\in F)(C(\vec{a}, \vec{x})\to \neg B(\vec{a}))] \tag{$\neg$* 2}
$$
or, finally,
$$
(\forall \vec{x} \in S)(\forall\vec{a}\in F)[(A(\bar x) \wedge C(\vec{a}, \vec{x}))\to \neg B(\vec{a})] \tag{$\neg$* 3}
$$
This last is the familiar standard form expressing linear independence ($\neg B(\vec{a})$ say that all the $a_i$ are zero).
