编辑：我已设法调整表格大小以适合一页，但我实在想不通为什么第一行偏离了方向并打印到了第二行。我也不明白为什么第一行的文本没有换行。
\documentclass[a4paper,12pt,twoside,BCOR=10mm]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[icelandic]{babel}
% \usepackage{t1enc}
\usepackage[T1]{fontenc}
\usepackage[dvipsnames]{xcolor}
\usepackage{graphicx,booktabs}
\usepackage[intoc]{nomencl}
\usepackage{enumerate,color}
\usepackage{url}
\usepackage{appendix}
\usepackage{float}
\usepackage{eso-pic}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{longtable}
\usepackage{acronym}
\usepackage{nicematrix}
%\usepackage[sf,normalsize]{subfigure}
\usepackage[format=plain,labelformat=simple,labelsep=colon]{caption}
\usepackage{placeins}
\usepackage{tabularx}
\usepackage{multirow}
\usepackage{adjustbox}
\usepackage{subcaption}
%\usepackage{subfig}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{external}
% Packages used for title page layout
\usepackage{tikz}
\usetikzlibrary{positioning}
\usepackage[pdfborder={0 0 0}]{hyperref}
% Blue color according to HÍ corporate design
\convertcolorspec{RGB}{16,9,159}{rgb}\tmphiblue
\definecolor{hiblue}{rgb}\tmphiblue
\setlength{\parskip}{\baselineskip}
\setlength{\parindent}{0cm}
\raggedbottom
\setkomafont{captionlabel}{\itshape}
\setkomafont{caption}{\itshape}
\setkomafont{section}{\FloatBarrier\Large}
\setcapwidth{\textwidth}
\usepackage{lmodern} % Use Latin Modern (instead of the default Computer Modern that is rendered using a bitmap font).
\usepackage{fixcmex} % To fix that Latin Modern large symbol math fonts has by default only one size: https://tex.stackexchange.com/a/621536
\usepackage{mathptmx}
\graphicspath{{figs/}} % Figures in directory figs
\usepackage[backend=biber, style=authoryear]{biblatex} % BibLaTeX used for references.
\usepackage{csquotes} % BibLaTex wants to have context sensitive quotes
\addbibresource{references.bib} % Name of *.bib file containing references
\begin{document}
\begin{table}
\centering
{\small\renewcommand{\arraystretch}{.8}
\caption{Spá um raforkunotkun, forgangsorka og skerðanleg orka. Almenn notkun að meðtöldum dreifitöpum og núverandi og samþykktir nýir stórnotendur (*Rauntölur). \label{table:Orkuspá}}
\resizebox{!}{.3\paperheight}{
\begin{NiceTabular}{lcccccccccc}
\hline
\Block{3-1}{Ár} & \Block{3-2}{Úttekt frá dreifikerfinu forgangs- og skerðanleg orka með dreifitöpum} & \Block{3-2}{Úttekt frá flutningskerfinu} & \Block{3-2}{Flutningstöp} & \Block{3-2}{Úttekt frá vinnslufyrirtækjum} & \Block{3-2}{Samtals} \\
& & & & & & & & & & \\
& & & & & & & & & & \\
& Orka GWh & Afl MW & Orka GWh & Afl MW & Orka GWh & Afl MW & Orka GWh & Afl MW & Orka GWh & Afl MW \\
\hline
2015* & 3.604,74 & 618,46 & 14.356,34 & 1.685,89 & 369,56 & 57,11 & 468,32 & 75,40 & 18.798,96 & 2.383,25 \\
2016* & 3.383,99 & 590,10 & 14.334,19 & 1.695,96 & 360,42 & 57,05 & 470,86 & 71,46 & 18.549,46 & 2.378,51 \\
2017* & 3.519,13 & 608,86 & 14.869,95 & 1.750,87 & 373,07 & 61,49 & 476,38 & 84,74 & 19.238,53 & 2.443,61 \\
\end{NiceTabular}}}
\end{table}
\end{document}
我试图让单元格内的长文本换行，就像下面的图片一样。但是，短标题应该位于单元格的中间，这就是为什么我把所有\Block{3-*}
不幸的是，我还没有找到我的问题的正确答案，但我会继续寻找。
答案1
您误解了\Block{X-Y}，它获取单元格的内容并将其分散到当前X单元格下方和Y右侧的单元格区域。因此，在您的示例中，\Block{3-1}{Ár}将内容放在Ár三个单元格深、一个单元格宽的区域中间。
更明显的是
\begin{NiceTabular}{ccc}
\Block{2-2}{a} & b & c \\
d & e & f \\
g & h & i \\
\end{NiceTabular}
它可以用于跨越多列或多行，
\begin{NiceTabular}{ccc}
\Block{2-1}{a} & \Block{1-2}{b} & \\
& e & f \\
g & h & i \\
\end{NiceTabular}
但它不会自动创建多个单元格，因此
\begin{NiceTabular}{ccc}
\Block{1-3}{a} & b & c \\
& e & f \\
g & h & i \\
\end{NiceTabular}
不会创建三个单元格a，然后创建两个带有 和 的附加单元格，b而是c将其设置a在三列的中间，然后将其放置b在第二列的相同位置，然后放置在第三列。相反，我们需要在第四列之后c手动添加两个空单元格。\Block{1-3}{a}b
所以你想要更多类似的东西
\begin{NiceTabular}{lcccc}
\hline
\Block{2-1}{Ár} & \Block{1-2}{Úttekt frá dreifikerfinu forgangs- og skerðanleg orka með dreifitöpum} & & \Block{1-2}{Úttekt frá flutningskerfinu} \\
& Orka GWh & Afl MW & Orka GWh & Afl MW \\
\hline
2015* & 3.604,74 & 618,46 & 14.356,34 & 1.685,89 \\
\end{NiceTabular}
不过，的多列变体\Block不会引入任何自动换行（单列\Block{1-X}可以，但在这种情况下也不会，因为您只使用l- 和c- 类型列）。这需要手动输入
\begin{NiceTabular}{lcccc}
\hline
\Block{2-1}{Ár} & \Block{1-2}{Úttekt frá dreifikerfinu\\ forgangs- og skerðanleg\\ orka með dreifitöpum} & & \Block{1-2}{Úttekt frá\\ flutningskerfinu} \\
& Orka GWh & Afl MW & Orka GWh & Afl MW \\
\hline
2015* & 3.604,74 & 618,46 & 14.356,34 & 1.685,89 \\
\end{NiceTabular}
可以通过设置块内的对齐方式\Block[<options>]{X-Y}{<content>}，但默认为居中，所以我认为我们已经知道您想要做的事情。
就我个人而言，我更倾向于\multicolumn使用固定宽度居中的列类型（如何创建带有文本“raggedright/centered/raggedleft”的固定宽度表格列？）
\newcolumntype{C}[1]{>{\centering\let\newline\\\arraybackslash\hspace{0pt}}m{#1}}
\begin{tabular}{lcccc}
\hline
Ár & \multicolumn{2}{C{4cm}}{Úttekt frá dreifikerfinu forgangs- og skerðanleg orka með dreifitöpum} & \multicolumn{2}{C{4cm}}{Úttekt frá flutningskerfinu} \\
& Orka GWh & Afl MW & Orka GWh & Afl MW \\
\hline
2015* & 3.604,74 & 618,46 & 14.356,34 & 1.685,89 \\
\end{tabular}
或者Zarko 的回答使用一些额外的花哨功能，效果非常好。
答案2
我宁愿使用tabularray带有库的包amsmath，booktabs和siunitx（它们加载同名的包）。除此之外，包ragged2e和选项的使用hspan=minimal可以更好地填充多列单元格中的文本：
\documentclass[a4paper,12pt,twoside,BCOR=10mm]{scrbook}
\usepackage[icelandic]{babel}
\usepackage[T1]{fontenc}
\usepackage{lmodern} % Use Latin Modern (instead of the default Computer Modern that is rendered using a bitmap font).
\usepackage[format=plain,
labelformat=simple,
labelsep=colon]{caption}
\usepackage{ragged2e}
\usepackage[dvipsnames]{xcolor}
\usepackage{tabularray}
\UseTblrLibrary{amsmath, booktabs, siunitx}
\begin{document}
\begin{table}
\footnotesize
\caption{Spá um raforkunotkun, forgangsorka og skerðanleg orka. Almenn notkun að meðtöldum dreifitöpum og núverandi og samþykktir nýir stórnotendur (*Rauntölur).
\label{table:Orkuspá}}
\begin{tblr}{colsep = 4pt,
colspec = {@{} X[l] * {10}{Q[c]}
@{} },
row{1,2} = {guard, m, mode=text, font=\scriptsize},
cell{1}{even[2]} = {c=2}{f, cmd=\RaggedRight},
hspan=minimal,
}
\toprule
& Úttekt frá dreifikerfinu forgangs- og skerðanleg orka með dreifitöpum
& & Úttekt frá flutningskerfinu
& & Flutningstöp
& & Úttekt frá vinnslufyrirtækjum
& & Samtals
& \\
\cmidrule[lr]{2-3}
\cmidrule[lr]{4-5}
\cmidrule[lr]{6-7}
\cmidrule[lr]{8-9}
\cmidrule[l]{10-11}
Ár & {Orka\\ GWh}
& {Afl\\ MW}
& {Orka\\ GWh}
& {Afl\\ MW}
& {Orka\\ GWh}
& {Afl\\ MW}
& {Orka\\ GWh}
& {Afl\\ MW}
& {Orka\\ GWh}
& {Afl\\ MW} \\
\midrule
2015* & 3.604,74 & 618,46 & 14.356,34 & 1.685,89 & 369,56 & 57,11 & 468,32 & 75,40 & 18.798,96 & 2.383,25 \\
2016* & 3.383,99 & 590,10 & 14.334,19 & 1.695,96 & 360,42 & 57,05 & 470,86 & 71,46 & 18.549,46 & 2.378,51 \\
2017* & 3.519,13 & 608,86 & 14.869,95 & 1.750,87 & 373,07 & 61,49 & 476,38 & 84,74 & 19.238,53 & 2.443,61 \\
\bottomrule
\end{tblr}
\end{table}
\end{document}